I would think a trip with just a preschooler would not be too difficult. Trip before last ds was 3.5 (and dd was turning 1) and I loved that trip with him. I think it was the first time that he was really aware of everything and had so much fun. Last trip I bought the Mac Volo stroller -- it was quick and easy to fold and had a shoulder strap. Just fold the stroller when you see the bus coming, throw it over your shoulder and get your dd's hand. At that age, you shouldn't need to carry around much, so you could throw your bag over your other stroller. I would imagine you will need to wake her on the busses if she's asleep because you wouldn't want to try to carry her and your stuff.
As for ex-dh, that would totally be your choice. If it makes you uncomfortable then I wouldn't do it. I have a friend who goes to Disney annually with her ex and his parents along with her and ex's dd. But their divorce was several years back and they've both moved on to other relationships.