Support for combat achievement diaries. It's probably the most anticipated piece of content a lot of people are waiting on right now so the question is when will it be added, rather than if it will be added.
As for your three questions, I'd like to throw my opinion into the mix.
What do we do about the slayer helmet kits already in-game?
It doesn't sit right with me knowing that the items that people associate with the Combat Diaries can be bought from the store. I wish we had some sort of say before this decision was made, but it is what it is. I wouldn't mind seeing everyone's slayer helms wiped like we saw with 3rd age sets back in the day. Giving people 75% of the store tokens back would be the way to do it from my POV. It would be better to cut the losses now and move on from it rather than looking at the helms 2 years later saying "ah but we should've made them available from CA's only".
Will Grandmaster Combat Achievement Tier be required for Completionist status?
This is a topic for another day, but I also wish we had a proper Completionist challenge or maybe a Completionist Cape (t) that would be genuine challenges. Perhaps if Omicron decides to introduce the actual Completionist Cape into the game then actual tasks (something similar to the OSRS Totus clan task list) would be added and it would be a proper end goal, rather than the current "do all diaries and get 3 200m skills" which is hardly a completionist challenge.
If none of the above is added, then I agree with your suggestion for this too.
Will kill count tasks be retro-active?
I agree with what you said, keeping it same as 07 is the way to go.
At the end of the day these are just my opinions and other than that great suggestion and great list I appreciate you taking your time coming up with the list of tasks.