As far as the trailer, just drop the seat down. Do you need the cross bar for side support? I like to leave the side flaps up as they protect the tires, as well as short flaps front and back to protect cargo without tying down or covering.

My Burley trailer gets adequate side support with the stroller bar on the back which keeps the bed area open, and gives me something in the back to tie onto.

My trailers without the stroller bar need a top bar to keep them together.

I just use the stock floor.

As far as the mirror. I find the bars are an awkard location to look at, so I prefer just looking back. But, there are mirrors that mount in the bar end plugs, so just cut a hole in the grips and mount the mirror.
Bicycle Handlebar Mirror Handle Bar Plug End Mount Rear View Flexi Convex Safety | eBay

Helmet/glasses mount?

There are lots of racks available, so I'm not sure what is best. One feature that I like is extra rearward support to keep panniers from flopping into the wheels. Also a center strip if you aren't also using fenders.

Somewhat like this, but lots are available.
Bicycle Rack Rear Topeak Super Tourist DX MTX Black | eBay