130mm rear is ample for may good road hubs and stretching it to 135mm is trivially easy. The stem should be fine, that's not that much of a stretch even for aluminum an should get back to its normal clearance. Those Suntour thumbies are desirable, you should have no issues selling them for what bar end shifters would cost you.
I really like the mountech rear derailleur. They got a bad reputation for failing but that's because they were not well suited for what they were intended, mountain bikes. On a road (and touring) bike they don't see the same amount of mud and grit flailing around. On that application they are solid performers with a good amount of chain capacity.
If you rewire the bike, the rear derailleur casing has a metal cup to fit the bit on the chainstay, make sure you don't loose it. A casing end cup is too fat to fit there, and a shifting casing without the end cap would fail in a hurry. For lack of that metal cup, a brake casing (the coiled metal kind) would work without an end cup.