your final question was will I have 29 players in my squad when the player returns from training and my answer was no because I currently have 27 in my squad and when he returns i will have 28. i don't know how it would work if I hadn't already sold the awarded player.
i'll add that some of the coaches aren't necessarily a bad deal because it allows you to train with currencies other than tokens and rests. one of the coaches (focused defending x33 min 6 stars) is for 4 days and it costs 90 greens, 108 blues and 270 reds. now i don't value those 270 reds that highly. a player that i would use (if i did it) is a 24 year old 6 star and this coach would take their tackling from 201% to 18-21% higher and marking from 204% to 16-18% higher. because of his age and being 6 stars so the rate of improvement per rest pack drops significantly, that's actually a bad use of 90 greens (yes I know blues and reds are involved but they don't have close to the same value). it's the 4 days away that really suck.
if I spent 90 greens (i'll give blues 33% the value of a green, so a total of about 125 greens) on the same player that I'm referring to I wouldn't take his tackling from 201% to 219-22% and marking from 204 to 220-22% and i would have to use a drill like 1-on-1 finishing to get the tackling as press the play is already maxed out for me and set piece delivery for the marking because all the drills that only train white skills for a DC are done for that player.
and my reds are over 750 now and increase all the time. it's the one thing that never stops going up for me. if they ever have a coach that takes just reds (even say 400), i'm doing it without a second thought.
i agree with the player going missing and that's the biggest reason i wouldn't train a starter. but we'll have to agree to disagree on valuing reds the same as greens. i just don't see it. i could see myself having 1000s of reds when i'm level 40 or something and not doing anything special to get them (like watching ads). greens and tokens are the most important things in the game and my primary purchase with my tokens are actually greens because i can count on 1 hand the amount of times i've purchased a player in the auction for more than 5 tokens. greens make the game go around.