How fast a player trains depend on how good he is being a fast trainer. ie. the more cones he receives from a set of 6x very hard drills with the lowest average on applicable skills within, the faster a trainer he is. This will be same as reflected in skills increase as well.
A dual-role player tends to have more whites than a sole-role player, as the player needs more white skills to excel in the additional role. As 15% skill points equal to 1% in player quality, a sole-role player will be highly likely to have higher whites than a dual-role player when they have the same quality, provided that we train both properly. Same principle applies with 3-role vs 2-role.
SA increases in the same rate as role. SA effect simply means adding another 20% onto the applicable skills, when activated. Eg. If a MC has a passing ability of 250, he will be performing like as if his passing skill is 270 when he has the playmaker SA and also in a situation where his SA is activated, such as taking CKs or making an assist. SA works best when we have our players’ best skills trained up as the highest white to maximise its effect.
2/3-role players could have extra benefits in acquiring the most ‘useful’ and ‘effective’ SAs with roles from 2 different lines or even 3. Eg. 2-role AMC(L/R)/ST could be very deadly with both 1-on-1 and shadow striker SAs; MC/AMC could have both playmaker & dribbler SAs to make them quite unstoppable in midfield.
(How useful and effective the SAs could be still depend on how we deploy the player himself, the supportive players around him, the kind of formations we use to maximise the triggering rate and effects of the 2 SAs, as well as the kind of playing style which suits the player and the whole team)