No Stupid Questions

IANAL. In a jurisdiction such as California, USA, there is a distinction between a criminal summons and a subpoena. The former is when that person is being charged with a crime, and the latter is when they are being ordered to give testimony to a criminal or civil trial. The difference as applied to the OP's scenario is that some treaties allow for criminal defendants to be extradited to a requesting jurisdiction to stand trial. But I'm not aware of any treaties that permit extradition to give testimony.

Indeed, in the 21st Century, when a person whose testimony would be greatly useful cannot appear in person for a tribunal, for whatever reason, the next best thing is to have them show up via telepresence. It accomplishes the goal of obtaining testimony, but does not imperil the physical or legal safety of the witness. In a civil trial, this is fairly straightforward to arrange, but in a criminal trial, there may be some concerns about the US Constitution's 6th Amendment, specifically the Confrontation Clause, that requires an opportunity for the accused to examine the witness.

But seeing as the title of the question specifically says "defendant", we might tackle the question of when someone overseas is charged with a crime. There are a few things that have to be determined, including jurisdiction, venue, and nexus. These are, grossly oversimplifying: the legal authority of a court to hear a case, whether the court is the most appropriate, and whether there is sufficient connection to apply the laws of a particular jurisdiction. Other locales may refer to these concepts differently, but the idea should be much the same.

Jurisdiction must be established over the person in question. That is, a potential defendant must be bound to the orders of the court, or else this is an entirely moot exercise. So obviously, someone physically within the judicial territory (eg California) is probably within the jurisdiction, but notable exceptions include diplomats and federal officers. Likewise, someone who was in the state but fled the authorities already pursuing them (eg fleeing a hit and run by driving into Nevada) did have jurisdiction for the relevant time of the crime, even though they're not physically still in the state.

Venue is the choice of which court should hear the case. Suppose we're using the hit and run scenario again, would the case be eligible to be heard in every single country that the motorist passed through while fleeing? Generally no, since the most appropriate venue would be the place where most of the facts occurred, which is usually where the crime occurred. That said, when it becomes impossible to try a defendant at the first choice of venue, the next choice can be used. This sometimes happens because a crime is so notorious that it'll become impossible to guarantee a fair trial. In California, a trial can be ordered moved to a different county, in this scenario.

But also possible is for a "foreign" court to exercise venue but applying California law. This is unlikely for criminal cases, but possible for civil cases, usually when a contract dispute specifies "the laws of California shall apply". In such scenario, another state court (or a federal court, whether within California or not) can apply the laws of California and render a verdict under the same principles and precedences that a California trial court would render. This even includes a process for "certifying" a question to the California Supreme Court, for matters where no precedence can be found, in order to get an authoritative answer for what California law says.

Finally, there is nexus, which is more about suitability of bringing the case. After all, certain crimes are still crimes even after fleeing the original place. If the same motorist from earlier flies out of Las Vegas airport directly to Edinburgh, Scotland, it may be possible to extradite them back to California if an extradition treaty is in place, or if the Scottish authorities approve of California's request to extradite even without a treaty. But in the alternate, the Scottish prosecutors might be able to charge the crime locally, even though it happened in California, and impose a penalty as though the crime occurred in Scotland. But this greatly depends on whether Scots law specifically allows such charges.

In the case that it does, nexus would be what tilts the decision to do the trial under California law in California, or Scots law in Scotland. If the crime turns out to be an attempted murder in furtherance of the Scottish Mafia (??), then a Scots law trial would make more sense. But if it were an attempted murder of ex's new beau and all events relate to the north coast town of Fort Bragg, then a California trial would make more sense.

There’s already a form of punishment from visa requirements meaning if the defendant has a passport from a “third world” country: their travel movement is restricted making any potential vacations a headache, but that alone doesn’t work if the defendant has a strong passport

I'm not sure if this is a thing in the USA. If someone has fled the jurisdiction, and has not been tried in absentia, then they have not been convicted, merely that they're still wanted. If anything, the USA would actually be very willing to welcome them to the country in vacation, because the moment they step foot off the airplane, USA jurisdiction comes into full force and that person can be immediately arrested and charged with the crime, assuming the statue of limitations has not run out. And given that many felonies have their statue of limitations tolled (read: paused) when an assailant is beyond the territory, this is a definite risk for someone who might be wanted by state authorities. Per the Full Faith And Credit Clause of the US Constitution, an arrest warrant from any one state has the same effect as though it came from the local state. So sheriff's deputies at Orlando Airport (which serves Disney World) could not decline a California arrest warrant, if they would have acted on a Florida-issued arrest warrant for the same crime.

(As a side note, so-called sanctuary jurisdictions do not run afoul of the Full Faith And Credit Clause, because courts in those jurisdictions are prohibited from honoring certain orders from any court, whether within or without the state. So long as the sanctuary legislation prohibits giving effect to both local and out-of-state orders, there is no constitutional issue. There may be other issues, but not one premised in this Clause)

The USA does not have the power to invalidate the passport of a non USA Citizen or non USA National, but since USA airports do not have international zones, this effectively might deny that person from being able to transit via the USA, such as wanting to go on vacation from their new home in Scotland to Disneyland Tokyo via Dallas/Fort Worth airport.

the defendant hasn’t even been to the country in person where the crime took place but are considered a suspect due to their interactions with those on the ground who executed the crime in where it occurred.

In California and the USA at large, guilt by association is not a thing. If they have not been to the territory where the crime occurred, the courts are unlikely to have jurisdiction and venue, and it's a very weak argument for nexus. The exception would be if this person has specifically orchestrated the local crime, directing it so thoroughly beyond a mere tangential connection. There are, frankly, few crimes that will meet this bar, but terrorism, conspiracy, murder-for-hire are a few that come to mind. And even then, the burden on the prosecution will be to prove a specific intent to aid in a crime within this state, as opposed to a lesser intent.

Consider that ransomware attacks do not care about jurisdiction, and whether the victim is in California or Copenhagen doesn't really change the nature of the crime. Such remotely-operated crimes are usually not charged at a state-level but are done by USA federal prosecutors, for whom extraterritorial jurisdiction is much more commonly applied, especially for computer crimes that are pursued by international consortiums.

But even this would still require finding the assailant, figuring out how to get them to a venue that can apply meaningful jurisdiction, and securing a conviction. Sometimes going into hiding is indeed a viable way to avoid prosecutors, but that's hardly a way to live life.

TL;DR: it depends