I have a bunch of different varieties of crabapples, grown from cuttings and airlayers, growing in the ground, and I live in Canada! I live a couple hours east of Guelph, and you're right it has been a slow start to the season, by at least a month.
I would avoid using a grafted tree for bonsai, most crabapples when chopped back that hard will throw sucker shoots even from below the graft, these will be the understock, which may not have the same disease resistance, leaf or flower type, plus the graft union unless you grow it into a larger trunk tree will more than likely grow into a ugly bulge.
I've purchased grafted trees only to use them for airlayers, so I plant them in the ground and pre-wire branches while they are young. This gives them some nice movement early on, which is something also that you don't often find with grafted trees. As cutting or airlayers they grow extremely fast after being planted in the ground after a couple of years to get established, even faster, I would say, than the grafted trees. My airlayers in ground have caught up in trunk size after only three years to the parent tree, and while this might seem longer than just a quick trunk chop, you would be looking at the same time to develop your tree.
Hope this helps.