Presume I would need to run Reverb lever on the left – would I better with a RHS lever upside down? Presume the cable run allows for this?

yes I found that is the best way. its neat and tidy out of the way under the bar. your thumb is naturally under the bar so all you need to do is extend your thumb and press the button.

Up to you? You can put it where you like. Most people (myself included) do what you suggest and run a RH lever upside down on the left.

On my xc bike with 1×10 setup I run a RH lever upside down on the left.
On my other bike which is 2 x 10 I cant get it to fit on the left so its on the RH on top of the bars. Ergonomically there’s not much in it- I probably prefer the one on the right but that’s probably because i’m right handed.
If I could fit it on the left side though I would just so it’s less likely to get damaged.

Yep, RH lever on the left, under the bars, You don’t want to have to bring your thumb up over the bar

Also if its upside down you can turn the bike upside down to remove wheels, fix drive trains etc. Safe in the knowledge your not going to bust the plunger – I seem to recall they are about £45 to replace.

I stuck mine in my thigh when failing to do a gap jump. Not sure which hurt most, the crash or the price.

LH lever on the left – it nestles nicely on top of an XT brake lever.
I’m used to going thumb above bar from using a RockShox lockout.