Hi, can you please post your routine - 3.5hr EASY I'm guessing is 2hr A time with a hoped-for 1.5hr nap? That might be part of the problem - at 5 months, A times are more like 2:15-2:30hr, so he could just be doing short naps because he's undertired.
WRT the dummy, if you want to get rid of it, do it soon. I would also say, though, that your routine will probably have to be closer to what he needs for that to go smoothly - it will be harder for him to settle if he's UT and harder for him to get through the transition and sleep a full nap.
Best thing you can do, IMO, is increase your A time.
WRT MIL - if you can get to a place of set naps by the time she's looking after LO, that might work? What's his temperament? For now, I'd drop LO off to her in the afternoon rather than the morning so she can do the CN rather than messing with the other naps in the day - better chance at a decent night for you, then.