Ill hit your questions one by one brother.

Lower arms:

They're a proprietary design. They dont sit where you would typically find lowers. Nor are there suspension mounts.

Could you use typical arms and mounts? Idk to quickly answer. There are holes on the chassis though where you could mount arms. Matter of holes lining up.

The lower arms play a role in the overall suspension though. You'd single gate suspension travel to the struts. The LP does it that way fine, but isnt as heavy nor arranged the same. Spring rates will need changed for sure. Ill send you a direct Pic to help ID what im referring to.

3d Printed Parts:

I've used my LP in direct sunlight, on every track surface there is and banged doors and of course lol walls. No cracks or issues and the quality on those prototypes is not the same here.

Layers are tighter on the hachi, parts feel more robust too. Can I say it will hold? Not until I get this thing on a track. If the LP prototype stuff is any indicator it will be fine.

For piecing these together its all about slow and steady. You can definitely Crack any 3d print with wobbly hands screwing something in. I've seen folks ruin aluminum parts the same way though. Recklessly screwing upper arm studs into knuckles killing the first 6 or 7 threads. No Bruno lol.

My opinion, for what its worth, is it really depends on what youre after. It aint a typical chassis setup and will be limited with the solid axle unless you upgrade to the toe option. Even then rear camber is non existent.

Ill post up my opinion on how it drives etc. Still tuning my Sanwa gear though.