Meerus intervening is just the same as when Zeno intervened and also i still don't think that goku will master Ultra instinct at this moment or at least i still don't think that this would be enough, i think they should bring up something new
Moro didn't seem to be out of UI Goku's reach when he was at full power, and MUI Goku is on a complete different level from UI Goku, so from what we've seen, MUI Goku should be on another level to Moro. With that said, it was mentioned that current Moro grew stronger from eating 73, but by how much is uncertain. Could also depend on Moro's magic, will he continue to absorb people's power to grow stronger ?
But even in that case, he'd need to get rid of Vegeta completely since his ability counters his absorption
And the editors also said before, they did too much with Broly and wanted to slow things down in terms of evolution in the following arc(Moro's current arc)
Which implied they wouldn't really make Moro as strong as Broly, in which case MUI Goku should be more than enough. With that said, whether Goku completely masters MUI this arc or not is uncertain, but he doesn't need to completely master it to dominate Moro.
He already unlocked it against Jiren when he was backed against a corner, and current Goku has much better mastery over UI than during that tournament since he can use UI at will now, except with a timelimit when going at full power. It's just the next step(MUI) that he needs to unlock / master in this case, and thus considering the arc has been resolving around Goku's UI training, i think it's natural to believe MUI will also come back, whether Goku completely masters it or not
His complete mastery will just play a role on if it'd be used to finish off Moro or not imo, with it depending on what they have been aiming with Meerus. Meerus is a character who had great influence in this arc, trained Goku to UI level, but also showed at times behaviour that make it seem like he'd be ready to disregard the angels rules of not interfering with mortals due to growing too fond of mortals, he had been questioning Whis about why they shouldn't go to help at multiple occasions, until Whis reminded him not to do that mistake, otherwise he'd die, so it'd entirely depend on if that plot point was developed to end up as Meerus deciding to sacrifice himself to save mortals or not