You might have to help the motor along through the thickest parts by turning the hand wheel. No big deal, since it’s only for a handful of stitches.
Yes, a free arm is of course helpful, but if you had to shorten the legs so far up that it wouldn’t fit, you would need to cut it off anyway. You can’t keep 15 or 20 cm of fabric inside the pant legs.
I have machines with both types, and prefer the front loading bobbins. But that’s probably just because that’s what I have been used to. I have had tangling and jamming on both, but only very sporadically, mostly due to low quality thread (especially cotton can be tricky if you get the wrong stuff. Polyester thread is more forgiving but shouldn’t be too old)