Getting from 15 to 17 ID is probably a 4 to 5 week adventure depending on the sublevels of the player's ID inside of 15.
Getting a player with say 1 Pa to 2 Pa will almost always take just 1 week. Getting a player from 2 to 3 will also almost always take just 1 week. Getting a player to 4 usually will take 2, unless the sublevels are low Or the HA/DR of the player are very low. So it's almost always benificial to get those first few, which is why i think a 1 in PA is only something you see when a manager is unwilling to train his player out of position (which can happen, not everyone likes to do that when they are doing their local club winning).
The ukraine u21 manager choosing 17 ID in many situations is not wrong. Against teams at the peak of talent in europe, you will find some 18-19 IS monsters there with just the basic training on their secondaries (they mostly will pick players that start with good secondaries, or a guard skill point total north of 25. In order for the Ukraine not to get beat against those teams, a player with more than 15 ID can be quite vital to shutting down the inside threat. It's more important perhaps than a well rounded player. However, it can be quite a bit of overkill when you face other teams that are mid level or lower in europe and have players with 13 or less IS. So again, it's really just a question of what you think you need.
As far as u21 versus NT development, while it is true that the NT would not start passing training until well after age 21, not all u21 starters need 12 passing, and in some cases playing a funnel approach to one target can be detrimental if he has high passing. I think an average SF passing of 9-10 is ideal, and depending on which style of offense you have for the game, it can be the same for the SG or slightly lower. extra PA on a SG is nice in look inside and more important even in low post which slows down tactics for quality shots. Only a handful of players are going to have trained PA quite early, and most of them are 7 potential players that cannot become long term threats for their clubs because of poor starting inside skills.