There are only a few production lines in China that make the cheap c band lnbs you mention. So more or less, they all come form the same place, and maybe even the same line.

DRO lnb drift was not mentioned much until somebody saw there was nothing new to sell and decided to introduce the cheap pll lnbs. I have cheap dro lnbs that are over 10 years old and they do not drift enough to notice. I have some commercial dro lnbs that are about 25 years old and they do not drift enough to notice. All of this is not to say that dro lnbs cannot drift enough to notice because it can happen, but usually the drift of a dro lnb will not be enough to really notice.

There were some videos at one point for the defunct WSI company that showed the l.o. being set on a production line of lnbs. The tuning slug you mention was being adjusted, so I doubt you would change much except mess the l.o.frequency up if you move that slug. Maybe look at little bit more, and compare with some of your other items and see if the lnb is as bad as you think?

The three transponders you mention on 101W are tough for many receivers or cards. I do not know what exactly those transponders are transmitting, but some of my fta receivers can get "confused" as to what is supposed to be on those transponders. If the lnb is actually drifting bad enough to notice, the blindscans will be producing different numbers for the transponder on almost every scan. I would suggest looking at other things before deciding the lnb is at fault. And it also sounds like you got those lnbs at a good price. If this is true, not much would be lost if you did turn the adjusting screw and it made the lnb useless.