I think it's plate on first then footrests you may have to drill bigger the holes in the plate hth

I've got the single seat with pillion pegs fitted but I don't think
I have that boomerang plate.
I'll have another look for you.

Edit:
I had a look underneath and then remembered what I did.
Firstly I do have that boomerang shaped plate (I'd just forgotten about it).

I had the single seat set up first and then I had to add the double seat just one time to bring
a mates 85 year old father for a spin.

Rather than spend hours removing the single seat undertray and replacing it with the standard set up
I just stripped back as much as I had to to fit the double seat.
I removed the carbon fibre tail piece to allow the seat to fit.
This leaves a half inch gap behind the seat which is a bit unsightly but I didn't care as it was
just for 1 day.

To get back to your point about fitting the rear pegs, instead of bolting them from the bottom up
towards the top I just got longer bolts and reversed the process.
That is from the top towards the bottom.
Or perhaps it was the other way around. It's so long ago I don't remember.
I am certain that I had to insert the bolts the opposite way that they were originally.

This removes the necessity of reaming out any holes bigger.

Hope this helps.