I don’t see this as a problem—and I’m not sure why any such insertion would be painful. There’s nothing wrong with originating characters for a project, and if the characters are good, there’s nothing wrong with introducing a Marvel Universe equivalent of them either. But most of those SHIELD Agents that you’re talking about are relatively faceless apart from a small handful, so all you’re talking about is baggage. Would Phil Coulson as played by Clark Gregg be a better character if his name happened to be Jasper Sitwell?
Yes. Yes, in fact, he would.
The rest of the point holds — Grant Ward, Melinda May and Leo Fitz wouldn’t be any better or worse if they were called Clay Quartermain, Helen Kim and Sidney Levine, fun though it’d be for a very small portion of the audience if they were.
But Phil Coulson would be a MUCH better character if his name was Jasper Sitwell. MUCH!
Yes. A thousand and one time Yes. Thank you.

