Exactly what I've found to be effective
Yeah DOMS tends to be worse as a noob. When you get more used to it, it can be deceiving because sometimes you may hardly even feel anything in the worked muscle the next day (especially if your training is low volume).
I wouldn't get too anal about it (better to focus on the basics), but in general, I believe that legs tend to respond better to high volume, and because of the size of them (and systematic demand), they need longer to recover optimally for size. Strength is a different issue IMO. I believe that you can get better strength by training them more frequently (and reducing volume), but better size with less frequency and high(er) volume.
1) Training a muscle before it's FULLY recovered is counter-productive (unless this is just because you're getting used to the routine/exercise or whatever). A muscle may not be fully recovered even after the soreness has gone. This is why leg training can be "tricky", because you may feel recovered, and you may get stronger, but it may not be optimal.
2) You cannot out-eat a badly suited routine (people say that bad routines don't exist, just bad diets...which IMO is too simplistic and wrong). Whatever type of training (e.g. low volume) works for you while not having to stuff your face to the point of puking, will work for you even more when you do over-eat. My gains were leaner when I got frequency sorted (especially for legs). Before they were either non existent, or too much fat. I made these gains with fewer calories than training lower body more often. Once again, to reiterate; if catabolism/training is too demanding, gains won't be great (they'll be little or not very lean) - got to get a good balance. This applies less for those with good genetics/on drugs.
a) Training bodyparts more frequently can be good for newbies, in that it builds strength/size faster (while they make newbie gains), but there comes a point of diminishing returns (for bodybuilding) when strength gets to a certain point (especially on the big exercises like squats/deadlifts).
b) Leg training is both metabolically and systematically challenging. The bigger "after-burn" means you have to eat more (speeded up metabolism). And more recovery resources are needed (hence more calories yet again). I don't see the point in burning more calories just for the sake of it (especially if there is no direct size advantage from it). If you are trying to gain, then more exercise other than just enough for building muscle is not going to be productive.
I believe for individuals like myself who have poor recovery compared to most, they need to use their gym time wisely (i.e. keep fatigue/'catabolic' activities to a minimum...which may mean lowering volume and having more off days than the average person).
A fast metabolism (whether naturally or via exercise) doesn't automatically mean leaner gains. There's much more involved than just metabolism when talking about lean gains/easy gains etc. A fast metabolism simply means that you have to eat more, when gaining, compared to someone with a slow metabolism.
A person with good genetics (whether fast metabolism or slow), will make good gains. A person with bad genetics (e.g. unfavourable hormone profile) will make gains that aren't very lean (even if they have a fast metabolism...because they HAVE to eat to the point of gaining).