- is there a limit to how many times can a character die and be returned? I'm guessing the penalties would be getting more harsh every time?

There's no real limit but five is probably realistically where the punishments get to be to the point that they're hard to work around, so officially the answer is no there is no limit, unofficially let's say try to avoid five.

- artificial humans. A character looks and mostly acts around 20 but has only technically been in this world for 9 years; would that be applicable or not?

As long as there is a strong case for them having an older mentality in their application then they can be apped - this is a case by case basis.

- would enhanced strength and regenerating abilities remain intact (as long as it doesn't work against the virus)?

As stated above: There will be no power nerfing unless the powers or skills are game breaking. It's a case by case basis. For example teleportation would be nerfed - we do not need characters teleporting to the Oasis Station, but short range teleportation is fine. With regards to healing and typical immunity to viruses - they would also be nerfed, nothing can heal away the Ira Virus. No characters are immune to the Pestilence (Ira virus).

Therefore healing can remain but it will be absolutely null with regards to the virus.

- if a character possesses what is essentially Magic Healing Blood, could it have effects like the virus spreading a little slower, OR even negative ones, for example the enemies becoming sturdier if they ingest it? In canon a few drops of it is enough to heal wounds on another person, obviously it wouldn't work like that here but it could be interesting if there were some downsides on the infected getting a hold on it.

All three of those scenarios are acceptable but it would never be able to purge the virus from someone. If you wish to make it into a player plot, you are welcome to as long as you give your fellow players a way to opt out if they wish.

- what if a character is missing a hand/part of an arm, and it's replaced by a highly functional prosthetic in canon? Would he be allowed to keep it, or have a new one attached etc?

They would keep it.

- it seems that weapons wouldn't be taken along, but what about jewelry? It's sort of like clothes, right. Rings, for example. Trust me it's important.

Jewelry is not taken unless it is large and bulky.

- MAGIC ABILITIES AGAIN. This time less OP, though. A character has "flames" (your shonen bullshit chakra/mana/life force/random power unit equivalent) with the ability to "neutralize", which basically translates to nerfing magical/energy projectiles, and numbing (or, at higher intensity, paralyzing) a human's body or its parts upon contact. Would that be allowed?

As long as it's not game breaking.

- What about roughly natural sized Magic Animal Summons, though.

Sure, but nothing is immune to the infection.

- If a character survives a "killing" blow in canon via administered magical aid (like... organ replacement magic... trust me you don't want me to get into the details), and it's implied that the presence of that magician is required to maintain it -- but the problem is fixable via eventual surgery. Would he survive arriving here, or should I take him from before this incident even though he technically lived?

This is up to the player.

EDIT: Also adding: LANGUAGES. Does everyone speak the same one, or do the earpieces translate it into something the wearer can understand? What about characters that speak several languages, could they maybe be useful in helping to communicate or deciphering something in the future etc?

There is a translator for known Earth languages i.e. Japanese, French, Italian, etc. But for languages not known to Earth i.e. Trigedasleng, Enochian, Lovecraftian, etc. they will not be translated. This feature can also be toggled off and on by voice command. Yes, language gaps can certainly be played with!