Like others, I drag my three-wheeler up stairs. Our school run includes a flight of stairs, and I happily bounce DS2 up it in the three-wheeler. With our McLaren fold-up buggy, I just carry it sideways, but that wouldn't work on those stairs, as they're narrow. DS2 loves being bounced up and down stairs, he laughs and laughs.
The goop does work, any bike shop would stock it.
Oh, I do have a problem getting my three-wheeler into small shops, but then I just leave it and DS2 outside (often with DS1 to keep him company).
If you don't walk a lot, I can see that a three-wheeler is a pointless accessory, but for people who walk, they're very sensible. DH will sometimes walk from Oxford Street to home with ours, which has got to be a couple of miles at least.
(hijack: bran, I'm glad you're enjoying Yi-Ban. Oh, there's a lovely New Vietnamese place in Lauriston village, just north of Vicky park, that's very enjoyable as well. Also, there's a fair in Mile End Park on Sunday, from 12-6, that looks v good. Not a rides-and-candyfloss-fair, but a community-music-food-fair.)