Technically possible and not illegal. However, if an immi officer flags abuse, you might be refused entry. Also, keep monitoring if the 45day program will indeed be available as needed.
Depends. Basically CNX offers the option of passing through customs and immigration there even your first touch down is BKK. IF you are on a properly routed (code share) connecting flight. Then your bags are being taken care of and routed through. No more than 1.5 hours connecting time necessary.
If this doesn't apply, I suggest 4 hours buffer minimum.