Can you explain how does adding an extra spacer will resolve the headset play? Also where did you purchase the spacer from?

I've been seen to 3d print moon shaped spacers so that i could compress the steerer column, avoiding the plastic thing that sits just on top of the frame, from rubbing on it. If it rubs, some of that compression torque is going into the frame, which will probably result in headset play. Generally, nothing on a bike should rub.
It probably sounds theoretical, but once you've built a few bikes and faced these problems, it makes sense.
As w most things, you learn by doing.

Make sure you torque enough & use carbon paste.
If it's a tolerance thing, then consider using a soda can shim, or some fiberglass cloth.
Also consider greasing the thread of the plug, especially if you've sweated on it. The torque shouldn't go into turning the screw, the torque should go into pushing the expander into the seat post.
Afaik, save for crazy exceptions, "all" seatpost slipping is fixable.

my er9 included all imaginable bolt lengths and adaptors for 140 to 160mm discs, something that costs extra and requires significant brain damage on Shimano. I think a very small fraction of the market cares about post mount (at least on the road setups), i wouldn't even want to pay for the shipping cost of bolts of another thread size. Shipping from China is a large portion of the cost of everything we ride in the West. 

I LOVE how they're trolling Factor logo & paint schemes.
The RS1 looks like various workswell / yishun / lightcarbon models, it's for sure OEM.
The SL1 could be a VB 177, bearing in mind VB is a broker, not a factory. It looks similar, but slightly different, for eg, to a long teng 266.
I dont follow gravel much.
Those boutique brands are basically a scam, it's incredible.

While I don't really understand the squared design of the downtube, I can not imagine that an average rider would feel any difference. Especially because I guess the front wheel will make the bigger difference in that area and It should kinda shield parts of the downtube from the air flow.

I think the rider position, clothing, ... will make the biggest difference in aerodynamics.

we know for a fact that rider & clothing matter more than the bike itself, it's about 80% rider and 20% bike, afaik.
That being said, a bus shaped tube is a bus shaped tube. Even without a wind tunnel, we know that tear drop / kamm tail shaped things are more aero than round things, or boxes... And we know that buses aren't aero. You wont find an aero frame with a wide, boxy shape like that, nor will you find a frame that touts its aero prowess with a boxy shape like that - until proven otherwise, ofc. Given it's an ultra light frame, and making frames of that weight at this price point is a new phenomenon, and that LC doesn't make any aero claims, i think it's safe to assume it's not aero. Whether it matters or not is subjective. But even if it "only" costs 5W at 45kmh, i try to spend my money diligently, and given how incredibly hard it is for me to raise my ftp by 5W, if i'm going to buy a frame, and if i can, i'm going to avoid buying one that screams "i'm a bus". It's a guesstimate. It's also an aesthetics consideration, as i dont find that boxy shape appealing.

Clear, thank you!

That's awesome! i did design and print a rear bento box for my propel a couple of years ago. it worked great until it disintegrated in a flat second. My advice: carbon fiber cloth or (cheaper) fiberglass cloth to reinforce everything (or pre preg to make your life easier, presumably). My box failed in the middle at some random layer line, which retrospectively, makes sense. It was more of an experiment than anything, i was phasing out the use of that bike already, so i didn't print a new one and didn't get around ordering cloth.

i like the ghetto TT front end rig a lot, well done!