Hard fork without non-mining full nodes = weak network?
It's not great, but non-mining full nodes are relatively inexpensive to operate, and if ordinary users start switching to that fork, some of them will bring their full nodes with them.  The network will get along okay with fewer.  If it really starts to be a problem, miners, exchange operators, and others who have a vested interest in the success of the fork, can fire up some full nodes.  

If ordinary users don't switch to the fork, then it's doomed regardless of how many miners choose it.  It doesn't do you any good to mine lots of a currency if nobody wants it.