165x42.5mm will not fit without the 120mm linkage, which the Vitus never used.  I was able to make rocker to seat tube contact without much effort.

Ha, I just responded to you on reddit.  Here's how my old Monstercross bike started life, I did a few rides with a fork and messed around with different tires, but mostly had it set up as a 29er gravel bike https://chinertown.com/index.php/topic,2837.msg20146.html#msg20146

Chainring clearance is limited by the chainstay.  MTB's have the wide tires that road bikes don't, so the chainstay flares outward where the tire is.  Bigger chainring means you need to push that little bump out rearward or inward, which means either making chainstays longer or limiting tire clearance.  In reality, 36t is probably the largest you'll find on a full squish.  The FM199 I built from had a bunch of chainring clearance, but it's a hardtail.  36t front and 10t rear is pretty tall gearing though.  Remember than 29er's are worth about 2t more speed than 700c, and 10t is 10% over 11t, so a 36/10t on 29er is similar to 42/11t on 700c.  Not enough gear to win a gravel race, but more than enough for XC racing and fine for most gravel stuff.  34/10t on my MTB's is enough to get around 30mph before I spin out and tuck.

Do you have an stl file for that bottle mount? I’d love to run a 2nd bottle on my L frame.

https://drive.google.com/file/d/1xMSEU1REAI_UqoaPSc10N6B0hA0j1nvj/view?usp=sharing

Yes you can use a zero-offset post, nothing wrong with that.  I will also add onto 2old2mould's reply that full suspension bikes sag more in the rear when you're going uphill, since the weight balance shifts rearward.  MTB seat tube angles are getting very steep because the terrain is often steep, which geometrically rotates the bike back and puts the saddle more rearward, and the additional rear suspension sag causes the saddle to shift rearward as well.  So it's not that 78deg STA is typically a good pedaling position, it's that 78deg STA ends up being more like 74, 75, 76deg when climbing. 

I'll add my experience, that I also like the steep STA of my FM936.  I also have a CFR707, and run the saddle in the middle of the offset post.  It's STA is still steeper than a traditional road bike, but not near MTB steepness.  But for me, it works very well.  Seatposts are cheap though, nothing wrong with trying out both kinds.  You can also just slam the saddle all the way forward on the offset post, and even though it's not safe to run on rough stuff, it'll give you an idea of what a zero-offset post would feel like.

Cite your sources.

https://www.sram.com/globalassets/document-hierarchy/service-manuals/sramrockshox-service-interval-counter-mat.pdf

SRAM does not have any service intervals that are calendar based.

I personally do fork lower and shock air cans once per year if I rode that bike a lot, or every other year if I didn't ride so much.  That would typically equate to the "every year" bikes being ridden around 1500 miles in a year, and the "every other year" bikes being ridden around 200-500 miles in a year.  Then I do damper service either every few years or whenever I feel that damping has changed.

I don't think I've ever seen freehubs that fit certain hubs but not made by the manufacturer, they are hub specific. Plus the engagement on a pawl hub is determined by the number of teeth on the drive ring inside the hub shell.

Is the only difference the linkage?  You use the same shock length and stroke?  Hopefully it would be OK with a 120mm fork (im fine with it slackening the HA a little I just wouldnt want to weaken the frame from the extra fork length)

The linkage only pushes the rear wheel further into rebound, raising the BB and giving more clearance from the seat stay bridge to seat tube. If you use the same shock, you'll have the same travel, you'll just ride higher. You'll need a 165x45mm (or remove the travel reducer during a rebuild from a 40 or 42.5) to the take advantage of the travel. A lot of people use 120mm forks on these, and with the longer linkage raising the rear end, a 120mm fork is preferred to keep geometry in check.