Hola amigo gallego!
I like your post! I think that those are the kind of doubts you get when you know you are going really fast but you feel you could go even faster. Often the extra bit is not so much about the gear itself but about the rider. Few little things that we do when we ride which are impossible to explain but that happen and influence in a good or in a bad way. That is why training means so much in slalom.
To question n. 1 and 2.. So if he is perfect powered up with the same sail size, same trimming, you will be disadvantaged as you are 5 kilos heavier. 5 kilos is quite a difference regardless the board volume.
to question 3. you should be faster.
In general if the guy is ok with 101, you should go for 107. Only when you get well overpowered and struggle to keep good control is the time to jump on the 90.
In my opinion the more volume the better as far as you ride confortably and with control.
Select fins are ok. Dbchet, are top!
97 litres as your sole slalom board might be too small for your 85 kilos and 7,8 sail.