I agree with you about the FH slice being an uncommon shot, it's usually a reaching defensive shot (squash shot) and sometimes used for a net approach shot (was more common in the past than today, but TE World Tour mode can start in the past

). This is largely the reason I thought seperating it into different talents would make sense, or maybe even only apply it to the backhand slice.
Having "Slice mastery" apply to both sides means the player gets a boost to their forehand slice too, when that player could almost never use it in real life and is a weak shot for them. In the case of a Karlovic type player, his BH would be given a much lower precision/consistency/power than the FH to control the effectiveness of his topspin strokes, but with slice mastery applying the same bonus to both sides, his BH slice would end up being worse (slower, less consistent) than his FH slice when the opposite would be true in real life.
Hope this makes sense and you understand what I mean

I also thought the speed bonus on slices was a bit high at the moment, but I hope you still keep in the ability to modify this slice speed between players - if these changes to slices can be done in talents, maybe they could be in attributes to give more freedom to choose slice abilities etc. Also maybe some talents could have a slider to choose the % skill or bonus rather than being an on/off switch that gives the same bonus to all.
In mods like the ITST mod where characters are designed to have variation in play style and balanced abilities, the more freedom over modifying all aspects of the player skills the better. Perhaps in order to keep the character sheet friendly to newcomer/casual players while allowing such finetuning for competitive play, you could have the current simple view as the default and an option to turn on advanced/dev mode for deeper/hidden options like in some other menus of the game.
On #3, thanks for the info about serve & volley - I've played this game for probably 1000+ hours and always held the key to serve until the ball contact, I thought releasing the key would do a b2 service in the middle of the box, I'll try this next time I play TE13

. The other part of my suggestion regarding doing the same for groundstrokes could be more tricky to implement, but it's basically being able to approach the net using a hop/carioca step to maintain forward momentum to the net through the stroke.