Apologies for interrupting a debate i'm not actually a part of but i'll attempt to answer these questions personally anyway.
1. We don't, quite simply. It could all go to shit and Moyes may simply not be the right man for the job. Do we conclusively know that he isn't now though? I don't think we do. Don't get me wrong, he has performed below par for even his standards since his inclusion, the transition shouldn't have hit us so hard even if it was going to take it's toll initially. However, he's had a year with the squad and to work out which players are in his plans and which players he intends to bring in. I think we all know this isn't Moyes ideal style of football, it can't be, otherwise he doesn't have a future even now. I don't buy anyone who draws back to his Everton days either; he has a much higher standard of quality in all aspects at Manchester United. It's new to Moyes as well though, he himself needs time to adapt and as a coach you have to adjust your philosophies in order to circumstance and as times change, and for me that's what we've got to allow Moyes to do.
The argument for me to sack him is has he done enough to justify spending our cash reserves and dedicating money to him for him to build his own side? Let's face it, we've still got a very solid group of players who are a lot better than what the league table suggests. Fair enough, it's not Moyes' squad but it's hardly a group of players that you would completely give up on and imply you simply can't do anything with them.
The question is though who do we bring in to replace him? Are there any top class managers available, and furthermore, would they stay here long term? What if they struggle to adjust and we have another season of mediocrity and we begin a certain managerial cycle a la Chelsea, whereby managers have to do the best with Fergie's has been's before being deemed worthy of a proper chance to grow.
I'm not going to sit here and tell you Moyes is the right man for the job. I couldn't tell you that, and it would be stupid. I also believe it's a risk allowing him to create his own side with his own philosophy off the back of his lacklustre performance this season. However, i think it's also very risky to sack him and search for a new manager, with no obvious replacement available and no guarantee that they'll actually do any better. I'd rather give Moyes a year or two more before realising that's probably not the best strategy, than throw enough shit at a wall every year in terms of managerial replacements in the hope that some of it might actually stick. Sorry for not addressing your second question directly, i'm sure i've covered it somewhere in there.