As to how effective Shin would be..........
Mmmmm....... I concede!
I think between the sickness and numbers.... they could wear him down to the point that he could believably fail to kill the main characters.
What I mean is...... betrayal is something that takes advantage of trust and I don't see him as somebody that trusts anyone but himself. He believes in "survival of the fittest" and it kind of reflects in his way of doing things. The Muhou school is basically a giant collection of former enemy schools that he now allows to serve him because they're....... strong enough to be useful to his goal. I don't think he sees his followers as allies so much as "Potential enemies that probably won't attack me unless they see it in their best interest, so I'll use them in the mean time.". Sort of like the Shishio/Usui relationship.