To be frank, I don't have experience with either training two-position for skills that are 100% at one position or training to sell, but I have the impression you'd maximize your sales income by going for maximum training effect, i.e. maximum speed for skills to pop - one-position training.
I also don't think it's necessary for new players to go for literally finishing their arena. Again, my team might not be the best example, but what I did was to expand gradually, in harmony with advancement through the leagues. That way, there was a part of funds always ready for investing into trainees or players to help me compete at a certain level. The ideal would be to get an impression of what arena size affords you good finances in the next tier league and go for that number.
Yes, I mean one on one. I've never trained handling in my 30+ seasons in BB, and I don't think I ever would, unless a player has significantly lower HA and needs to be finished off in terms of training.
I agree with what you said and just top it up I want to say that balance in developing arena and players is best way to enjoy this game. If you spend all of the finances on arena first and you have nothing left for youth players and some competitive games it will be kinda boring to play. However I agree that firstly you need to invest significant amount on arena, buy few players and try to improve profits and then continue to reinvest.