On the fitting JohnR, Jan Heine at Rene Herse recommends fitting tubes first as standard practice with their tyres, which have very flexible carcasses and are notorious for being hard to set up. Except that he doesn't do any miles on them with tubes - just fits them, inflates the tyre till it locks to the rim, deflates, unhooks one side, removes tube, refits the side, add sealant, amd inflates. Seems to think it always works.
My limited experience suggests that a folding tyre needs time on the rim with a tube in order to get rid of the kinks in the sidewall where the tyre had been folded . A gap of a few mm caused by one of these kinks will let the air out faster than it can be blown in. Warmth and sunshine might help in this process which, in my case, are better obtained when the bike is outside (depending on the weather) than being in the garage. I had found that one 27 mile ride wasn't enough to get the tyre into shape but it seated OK after another three rides.
I've often had one side of a tyre seated on the rim but there's a gap on the other side too big for my inflation system to overcome. I suspect that some tyre - rim combinations work better than others.
