The negotiations for his testimony are a defense tactic. They want none (if possible, highly unlikely), then written questions with written responses. Anything else is a huge L for his defense team. If he refuses, Mueller can issue a GJ subpoena. They can then respond with "he'll plead the 5th to everything". That's where it'll get fun. Can't have the optics of President "No Collusion" pleading the 5th to questions without it looking really terrible.
My prediction : Mueller has to go the GJ subpoena route. Trump declines and fights it based on some twisting of privilege (executive maybe), and it gets legally messy.
Having said all that, the news that they're now looking to question him is YUGE. This is the clearest indication, and short of a gold lettered invitation, the closest you'll ever get, that the president himself is a target of the investigation. You don't talk to everyone else, roll up subordinates, and then interview someone at the end if you aren't going after them. Cheeto Musolini is likely shitting in his depends right now. And rightfully so.