So tell me, what DV & HN should have SF in begining U21?
I think DV&HN very good pops when you train samoething else(as secondary skills). In U21 you haven't got time to waste time to train 1on1. Also when I watch a players from U21 almost always good DV+HN have PG and SG.
Good DR+IS can make a SF unstoppable inside player. Same with JS+JR+HN - great outside SF. DR and HN are supportive skills mainly for JS, but also for IS. DR is "how often will your player shoot" and HN is "how is your player going to shoot". If your player has great JS and bad HN/DR, he will depend from other's players' passes and won't be able to do much on his own.
Plus 1 on 1 is a great tool to speed up the training of OD, JR and PA.
So you have 21y old SFs with nice salaries, but they didn't get invited into their U21. This should tell you something about your training.
Oh and BTW, let me tell you something about handling since you obviously aren't a big fan of it: in my opinion, three pops in Handling is same as one pop in JS and one in JR. So I value HA quite high on my list.