mtGOX was insolvent because they lost all their assets, thats not the case for bitfinex as far as we know. That is independent from tether itself, it can happen to all exchanges.
Yes, precisely the sort of information an audit would reveal. Curious, then, that despite their repeated pledges for robust auditing they first declared they'd engaged a firm to do those audits that didn't actually do audits, then terminated their relationship with the second, legitimate auditor.
An audit of tether would not show you if bitfinex is solvent or not. Even if tether as a project is a total failure, bitfinex could pay at length for its shortcomings.
Plus, they made most of the money on the play itself.
So was mtGox.