The bolded part is where crossing in FIFA really breaks down. When the ball is in the air, there seems to be an "on the rails" aspect to the game, where every defender near an attacker gets suctioned into a 50/50 jump attempt. If you notice, you do not have control of the attacking player in these instances...even if the tick mark indicates that you do. You can't release from the challenge and try to get a step ahead while the ball is in the air.
I don't really think it's a problem with your playing style, I think it's just the nature of the game. Year after year they try to make aerial challenges more organic but it does not ever seem to be truly organic. It seems to me that the AI decides who's going to win the ball while the ball is in the air, and nothing you do in terms of controlling the attacker can change that.
To briefly touch on some of your other questions, though I doubt my advice would be very useful because you're probably doing it already...
Far post crosses work best...defenders in FIFA are usually single man marking on the far post.
Aim at a teammate if you can. Especially on manual crossing, I've found my balls aimed into space usually do that just, and nothing but..until they hit the endline. Even nearby attackers don't have the AI to go after it if it's not crossed directly at them.
Use X and not the finesse cross. The extra pace on the ball usually translates to a sharper header.
I use L-stick to aim as soon as I have made my last touch on the ball. So, if I'm on the wing I give it one last full tap ahead, press X for the desired power then start my L-stick positioning, before I make contact with the ball. I'm trying to recall right now if, in some instances, I actually use L-stick BEFORE and during X, but without the controller and actually doing it I can't picture it precisely.
All in all, you're probably doing as best you can given the current game limitations on air challenges. I know that I've adapted my offense to ground crosses/thru balls with Y, and that works a lot better with the game engine. It means a more counterattack oriented game, but I'd rather have that than trying to beat a bunkered in D with crosses that are unwinnable in the air.