Wire your ground from one ground on the I-PAC, daisychaining to all the COM leads on your microswitches, ending with the chain on a second ground on the I-PAC. If there's a single break anywhere in your daisychain, all your buttons will still work because each will have a path back to ground.
You don't *need* to connect up to the second ground on the I-PAC, but it does give you a measure of redundancy.
---- saint