Re: why use Mugetsu??
I'm aware this is an old thread but I wanted to post my opinion.
My interpretation of the reasoning behind using the Final Getsuga Tensho (FGT) was that Ichigo was at that time stronger than Aizen in every way. However, Ichigo was smart, he understood that the Hogyoku would aid Aizen in his quest for more power by transforming him continuously, increasing his power while doing so. I believe that Ichigo truly thought that the FGT would be powerful enough to end Aizen.
In terms of Ichigo's base power (Dangai) when he was fighting Aizen. You could tell that when Aizen transformed during their battle that he had begun to catch up to Ichigo. It's likely that Ichigo was also aware of this and ultimately decided to use a technique that he knew (or atleast thought) would be strong enough to over-kill him or at the very least damage him to an extent where the Hogyoku could no longer repair him.
Thats my 2 cents anyway.