What exactly does the phrase "same character" mean in game terms?

What am I looking for on my sideline to figure out if it is the 'same' character?

If for instance I have an ironman and ironpatriot, and an iroman and warmachine, and both split into AVM tony stark, and IIM Iron man (the ancestral spirit one who is real-name rhodes)

when I try to merge how do I figure out which AVM stark was which and which rhodes was which?

If you are using a game effect that is checking for specifics such as which character is which, it is both you and your opponents responsibility to keep track.

While it probably would not be that big of a deal to which character was which as they sit in your Sideline, but the characters on the board must be the same ones that were brought in using the ability earlier if the point value is a factor.

so, to answer the first question, is "same character" only "same name, number, set, point cost, etc." or would it also include any figures that replaced the figure that was split into?

For instance if I split from warmachine/ironman 400 pts into FI300 iron man and IIM38 iron man

if FI 300 might reverts into another iron man figure, can I still merge him with IIM38 to get back my duo?

Yes, but because when it was replaced via Revert, it retains the effect of being the character that split already... you could not merge with a different IIM030 or FI300 if the points for merge did not allow it since they were not characters that previously paired for split.

Will the character that you replaced with still need to on their own be a 'qualifying character'? or since they split off we get to ignore that?

(for instance, warmachine/iron patriot splits into some tony tark, and IIM34 james rhodes, rhodes ancestral spirits into Iron pharaoh, can iron pharaoh still mere with stark back to the duo? despite not having the correct true name?)

They would still need to be qualifying characters via the Name chart. The other restriction only bypasses the point restrictions for Split/Merge.