Time after time we've told you to get the basics down, but you won't listen.
Denis, as I'm sure you are aware there are thousands of members here and each one has 2 unique opinions about any given topic. The op is getting advice from dozens of users, not just from you. he is listening, intently but unfortunately he is getting way too many conflicting sets of instructions on how to best continue his training and education in the shooting arts.
he's got guys like you and me, who have been shooting long enough to know what is detrimental and what's not.
then he's got the guys that shoot about ten rounds a year(usually just to make sure their rifles are zeroed before hunting season.
then he's got the guys that post simple 1 sentence instructions that make no sense to anyone but themselves for the simple reason of getting their post counts up.
then he's got guys who don't understand just what Mosin-Marauder's circumstances are, what stage of his training he's in and offer advice out of context.
then there are the guys that learned a certain way and they are going to offer advice to that effect because it was how they learned.
I agree with you on much of your points, benchrest until he gets his skills down and knows what his rifle is capable of in the best of conditions. I've seen M-M's groups with his K31 and they appear to be much better than the mosin leading me to believe that his skills, at least in short spurts, are greatly improved and he may be getting to the point that he can start working from real world shooting positions, as I'm sure he will be required to do as part of his 4H club. however I will continue to say that he shoud do so either once he has established a good accuracy load for his mosin or while using his K31 or 22LR, although given the poor availability of 22, it may be in his best interest to use the K31.
EDIT!
there should be no shooting free hand, shooting ALL DAY, no Shooting from the hip, no calling your shots, no timed drills, and no further modifications of the rifle, at least for the time being.
a sling is not going to help from a shooting bench unless he is shooting elbows down with no sandbags which may be a good for transitioning from bench rest to field positions but not something that is needed at this juncture.
Another Edit: M-M
have you spent any time in dry fire? although it's no substitute for live fire, it is a good way to work on trigger control.