The Iowa flag website is indeed a nice one. Too bad more states don't have something of that sort.
As for Samuel Bell's activities after PG I will have to let them go. I barely have the mental capacity to deal with one battle at a time and sometimes I am not so certain about that.
So, based on Columbus Gray's letter, and the absence of any contrary evidence regarding Bell, it appears that Geoghegan was indeed at PG and that he temporarily succeeded Pleasants in command of the regiment.