Arteta will win manager of the year 100% guaranteed. No matter if Pep wins the treble.
There are a few genuine contenders for MOTS. Pep is the best manager, but City winning the league is a fairly 'normal' achievement now. UCL and FA cup aren't taken into consideration. Would it be fair to argue that:
1 - Eddie Howe breaking into the top 4
2 - Unai Emery turning Villa's season around
3 - Thomas Frank pushing for Europa League places
4 - Marco Silva finishing in the top half
5 - Gary O'Neil comfortably keeping Bournemouth in the league
6 - Roberto De Zerbi's progress with Brighton
Have been more deserving of MOTS this year?
If Arsenal win the league then Arteta deserves it too.
I wouldn't swap Pep for any of those managers, but they have done brilliantly this year.