A lot of questions there
Firstly, you have had him a week you say? You should have waited at least one week to attempt feeding him, to allow him to settle in. Maybe don't attempt another feeding for at least another week. What methods have you tried to thaw and present the rat?
And do you know if the store had successfully fed your BP 3 consecutive (in a row) times with f/t rats? If not, they shouldn't have sold him to you.
The humidity is fine. They are not corn snakes; they require higher humidity, 60-70% range typically.
The yawn is probably nothing. They sometimes yawn after feeding to put their jaws back in place. It's when you have consistent yawning that you should be on the worried side and checking for other symptoms of RI.
As for the temps, do you have a heat lamp or heat emitter on top of the UTH and thermostat? That will help with the ambient air temps for your BP, which should never dip below 75F, but should stay around 80F!!
This can also dry out the enclosure, depending on the humidity in the air in general.
Substrate like aspen will dry out the enclosure. If it is too dry (ie. below ~40%) there are many other types out that that hold humidity better and tend not to mould (for example, cyprus mulch) if aspen doesn't work for you.