Lets say I draft a WR rookie with 79 speed. If I switch him to RB position before training camp, will that increase his max speed? If not, is there a way to increase max speed?

No, WR's are the fastest players on the offensive side of the ball. Moving them to RB will only decrease their speed because they'll gain weight to hit the average weight for RB's. The only thing you can hope for when drafting a 79 speed WR is that he is overweight and will gain speed. If he's overweight, he'll continue to gain speed while losing weight to meet the average for WR weight. Speed changes are always due to weight changes(weight is tied to positions).

No. That's incorrect. You have it backwards.

So if I switch him to RB.... he will gain max speed?

No, he will lose max speed. The max speed for WR’a is 95. Rarely they’ll hit 96. For RB’s the max speed I’ve seen is 92.

Wrong. There are several WRs with 97 speed and I found 1 RB with 96 speed in German NFL and even 1 RB with 97 speed in Tri-State League..

but all those examples are underweight players for their position so the speed will probably hit 96 at WR standard weight

NO. He will get slower.

RB Cletus Kim was originally a WR and then converted to RB. He’s underweight even for the WR position which means he’ll eventually end up losing max speed as he gains weight. As he progresses on a team, he’ll end up losing speed until he hits the average weight for RB’s.

Might be right. However your claim max speed for WRs is still wrong.
Btw Cletus Kim made only 1 Season for Washington as ... RB !!!