91LS400vgb

I faced a similar issue and was able to unscrew them using a needle nose vise grip pliers.

I drive mine onto ramps for easy access to the drain plug and oil filter - I think it's a bit too tight, otherwise.

Oh, sorry - I should have said left side - yes.

Yes, you can change the oil without removing the engine cover. The pan drain plug is obviously accessible. To get to the filter, just remove 2 bolts on the (--corrected after skraggy_uk note below) LEFT side rear of the cover. It is designed so it can be folded down to access the filter.

Seems like a common problem. I corrected mine by applying some clear silicone around the sensor cap.