There's every chance here that this deal represents a way out for the US as well, and that it will be kept for that reason, but if the US government still wants him to stand trial, a plea deal and the risk of a minor diplomatic scuffle at a point in time where the UK parliamentary election will overshadow the case in UK media isn't going to stop them.

Keep in mind he doesn't have any support from the UK government - they'd rather be rid of him -, and the current UK government is almost certain to be out of government shortly. It's unlikely there'd be more of a diplomatic incident than a slightly stern letter.

I think he has reasonable odds - this case is likely at this point mostly just a nuisance for everyone involved except Assange himself. There's nothing to be gained, other than perhaps for some overzealous prosecutor. But I also would not be one bit surprised if something was to happen.