Yeah, sorry, I intended to post it with some more documentation, but had to leave for dinner. Sorry for the cryptic bits.

The 2.6 is arbitrary but the increase is due to Whirlwind Attack. I pulled some proportions out of thin air to estimate how often the ranger could get more than two enemies adjacent to them, since in that case they get as many attacks as there are enemies they can reach. I put in 1/8 of the time for 3 enemies, 1/20th each for 4 and 5, 1/40th for 6, etc. Taking the average number of adjacent enemies over 2 based on these arbitrary numbers gave an extra 0.6 attacks on average.

This is the proportion of the time the attacker hits at least once. Since colossus slayer is something you can use once per turn but declare it after you hit, in order to figure out how much damage to expect from it, you need to know what proportion of the time it gets triggered. To be really precise you also need to know how many crit dice you roll due to that feature. This is the most complicated formula in the spreadsheet: I assume you use it on the first hit on your turn, which may or may not be a crit. So the chance that you get a crit on the attack when you use colossus slayer, if you have two attacks, is p(crit on 1st) + p(miss on 1st) *p(crit on 2nd) + p(miss on 1st) * p(miss on 2nd) * p(crit on 3rd).

I'm realizing that I didn't integrate this at all with whirlwind strike, so colossus slayer crit damage assumes the ranger only ever does two attacks. But it's going to be a tiny difference from the reality.

The chance of an AoO is just an arbitrary parameter. I thought 1 in 3 rounds seemed high. But that's just my subjective sense. Reactions/Combat is the number of reaction attacks made per combat, as you had it: it's just the chance of an AoO times the number of rounds in a combat.

This is just the average damage your next ally will do with their first attack, not counting their ability modifier (since that's level specific). I just put in a lowball estimate of 3.5 assuming an ally using a d6 weapon with no fighting style. Realistically you're probably using a different maneuver if the beneficiary of your distracting strike would be blowing it on such a crappy attack. If there's a rogue going after you it could be much much higher.