Pretty sure Jericho has said he's not going to be at Mania. Unless that's changed from a few weeks ago?
IIRC, that statement was prior to Jericho's latest return. I imagine he was just trying to muddy the waters. Nothing is certain, but I expect Jericho at Mania.
Ambrose vs Jericho would be a miscalculation though. As far as I'm concerned, it takes a great deal to get these large stadium crowds invested enough in a match for it to register on television. Love both guys, but putting them together is likely to fall flat at an event like this and would be a waste of two men better served elsewhere on the card. The arenas WWE normally runs are more conducive to getting a worthwhile reaction for this match. I'd pass on Ambrose vs Jericho for the time being.
Jericho's shown some fire in his latest return and really seems to relish working with Styles. I'd find a way to stretch their angle out until WrestleMania with both guys getting more short-tempered after each outing. AJ's going to be hot enough in front of that crowd to negate the relative staleness of Jericho. 2/3 falls or some similar stipulation to blow this off. Milk every last bit of pro wrestling out of Y2J.
But as to the topic at hand, there are only a few situations worthy of Ambrose at this point. The last couple Raws have made it clear that Ambrose vs Lesnar is a pairing with some sizzle. They aren't locked into Wyatt vs Lesnar yet, so it wouldn't be jarring for them to call an audible and go with the organic build for the cockroach Ambrose vs the nuclear holocaust Lesnar. Wyatt can find a home elsewhere on the card and square off with Brock at a later date.
Failing that, Ambrose should be in the main event. Three way with Reigns or one-on-one with Hunter while Reigns finds himself against Taker or Lesnar. The Mania crowd would gladly take Dean over Roman. I foolishly believe that should count for something.
I doubt any of these scenarios will occur. But those are the best options.