MMZR uses our proprietary engine - the Immortal Engine. Most of everything is written in C#.
For the lack of a better answer, when it's done. We wish to take as much time as is needed so as to not put out a broken or half baked product.
Any ports that may or may not be made will be done after the core game is released.
A PVP gamemode is planned to be released after the main game is complete.
If we told you, we would be spoiling the plot of the game.
We cannot confirm or deny the appearance of any returning characters.
While amnesia was a theme in the Mega Man Zero games, it was made clear in those games that Zero gradually remembered his old life.
To give a few examples, where the first two games showed a very quiet and often passive Zero, Zero's personality in MMZ3 onward is much more akin to how he behaves in the Mega Man X games - confident and talkative.
Furthermere, Zero's "I never called myself a hero" speech at the end of Zero 4 indicates he would remember enough about himself to know how he does and does not act in the games prior to the Zero series.
Finally, page 21 of the Mega Man Zero Official Complete Works outright states that Zero "recovers his memories as he battles Neo Arcadia".
Zero has never been a character who stays dead for long.
In Mega Man X1, he served an "Obi-Wan"-like role for X, being a representation of how strong X could become. By the end of that game, X becomes as strong as Zero, and Zero sacrafices himself so that the two can defeat Vile. One could argue that at that point, Zero had served his purpose and was no longer needed. However in the next game, Mega Man X2, he was resurrected.
The later Mega Man X games, especially X4 and X5, further expand on Zero's character and role in the Mega Man universe. He is revealed to be the host of the "Original Virus", being responsible for the Sigma Virus's creation and spread. Zero has to deal with these implications, feeling his very existence may be a liability, while also trying to figure out who he is.
With the events of Mega Man X5, he died once more. In his final moments, Zero states that he as to die to "restore the peace". However, Mega Man X6 brings him back, waiting only three weeks in the game cannon before resurrecting Zero. While Mega Man X6 is not a well remembered game, Zero's return is generally appreciated by fans who have played Mega Man X8 and the Mega Man Zero series.
Even after Zero's death in Mega Man Zero 4, Inti Creates still chose to bring him back in Mega Man ZX in the form of Model Z. It is stated in the game that biometals contain "the consciousness of those legendary heroes". Despite the Guardians, Zero, X and Weil perishing, they are all brought back in ZX. The only difference between ZX and prior titles is that they are not in a leading role anymore.
With all that being said, the purpose of Mega Man Zero: Rezurrection is not to invalidate the events of Mega Man Zero 4, but to expand on what happened between Mega Man Zero 4 and Mega Man ZX. The game takes place a hundred years after the events of Zero 4. In that time a lot has changed, and we see an opportunity to explore what the characters of the Zero series did during that time. This game is as much a story about those characters as it is a story about Zero and X.
For what it is worth, the producer of the Mega Man Zero and ZX games, Takuya Aizu, to paraphrase, believes that Zero could plausibly have adventures during the events between Z4 and ZX. So in a sense, even the people behind the Zero series may be willing to bring Zero back.
Mega Man Zero Rezurrection is a non-profit fan game.
MMZR and Iliad Games are not associated with Capcom or Inti Creates.