a two bowler in league.

Can this bowler be considered a right hander and convert spare with one right hand only?

yes

Yes, they can switch from two hands to one hand.  As long as the one-handed throw does not change (always righty OR always lefty), they are legal.

That is an awful rule.  If the use two hands for 1st they should have to use both for spares.  Or all left, right or both hand rules should be removed.

AMEN! Why does USBC care what happens before the foul line anyway? 

Baseball's rule for the batter is that they stay inside the box. Our rule should be that you stay behind the foul line. With today's THSs having a "slight" advantage because of the release hand isn't going to amount to a hill of beans.