mounting panels to treated-wood frame

I am proceeding with a 4+kW ground-mount installation built on a base of treated wood. The base will be four concrete piers, supporting two beams (two-ply 2x12) with 2x8 purlins supported between the beams. I'm planning to use MCA-treated wood, which supposedly less toxic to metals (and people), particularly the aluminum in the panel frames; also, around here at least, it's easier to find the 20ft lengths I need in MCA (and #1 grade). The purlins will be perpendicular to the long edge of the panels, so each purlin will intersect the panel frame at two of the four mandated clamping zones (roughly 4" long and centered about 12" from the panel corner). It's a 4x4 array of Talesun TP660P panels, in landscape orientation.

So, I'm trying to work out the details of attaching the panels to the purlins. I plan to use 5/16" stainless-steel lag bolts with washers. Things I can't quite decide:

1. Should I use 18-8 or 316 grade stainless ? I'm sure both are strong enough and resistant enough to general corrosion. But I wonder if the more expensive Type 316 is more resistant to possible galvanic corrosion (at the interface with the aluminum panel frame). Talesun doesn't seem too worried, in fact, they specify SS (without mentioning a grade) if bolting the panels using the thru holes, and I believe all the makers of metal racking systems use SS for the clamps. IronRidge says "the high quality stainless steel used in the IronRidge UFO [clamping bolt] does not cause significant galvanic corrosion, even in extreme environments"; I wonder if they're talking about 316 instead of 18-8 and if indeed the former is more resistant to galvanic corrosion ? Anyhow, my environment is not marine, just probably not "extreme".

2. Should I use spacers around the part of the lag bolt that is tight between two neighboring panels ? Spacers are inexplicably expensive, but I scored some SS tubing (not sure what grade) at the local scrap-metal yard (a consignment store for guys that is exactly 5/16" I.D. and 1/2" O.D., so I figure I can make spacers myself with an abrasive wheel on a miter saw. Might be a bit of a bore to clean 'em up after I cut 'em though, so I wonder if they're really needed ? I worry that without them, the rotating bolt (as I tighten them) could bugger the edges of the frames; just seems cleaner too.

3. Is 4" a good length ? I figure that will bury about 2.5" of bolt (4" minus the 35mm panel thickness and the washer). I imagine less would be theoretically strong enough, but 4" isn't that much trouble or expense.

4. Are washers actually suitable as clamps ? The installation manual says clamps should be 2" long (along the edge of the frame). But the IronRidge UFO (clamping bolt) just clamps the panels down with a washer, and one a bit smaller than I'd likely use (probably 1.25", the 1/2" spacer if I use it, plus 2x the 11mm width of the edge of the panel frame).

5. Should I attempt to ground the panels with the washers ? If I could make contact between the washers and the top edges of the frame, I'd only need to run ground wire along the top or bottom edge, connecting to 4 panels total. Not sure where to get the washers I'd need, with teeth on the underside to gouge into the panel frame (like on the IronRidge UFO). Otherwise, I have to buy 16 grounding clamps (like the $4 Tyco 1954381-2 that Talesun recommends, and a lot more wire).

6. The PE who stamped drawings of my frame was at pains to tell me to paint the wood, so it doesn't warp and then potentially damage the panels that are bolted to it. Wise words I guess. Except generally treated wood is pretty wet when you get it, so you need to let it dry out before painting, and it warps when it dries out. Maybe build the frame, THEN let it dry out, then paint it and mount the panels ?

TIA !!