Fixing a built-in cupboard

[Deadeye]

A "I think I know but I'm terrified of screwing it up at the 11th hour" thread.

I was asked to make a cupboard to go in the alcove of the through kitchen/living room of the "young people just in their first home".
As they're in Sheffield and I'm not, I've built it to their measurements. We will see if this was a mistake, but it's mm perfect to those numbers.

It's a simple frame with 2 pairs of doors, no central stile, horizontal rails at top and between the upper and lower doors. The shelves sit separately on rails on the sides of the alcove.

My question is how to fit the frame to the walls.
Option 1. Drill through the jamb/frame (50mm) and use frame fixings into the wall. The screw heads will be hidden as they are in the angle of the closed door. Shim out any variations in plastering so the frame is true and caulk over.
Option 2. Fix a batten to the wall (framing screws) and glue the frame to that; caulk the gap.

#1 is simplest, but means 50mm of screw is just in the frame not the wall and might be harder to keep true?

Opinions welcomed!

 

[Amplidyne]

Steel brackets?
What about an architrave to cover up the gap?

 

[Deadeye]

Steel brackets?
What about an architrave to cover up the gap?

Yes....hmmm...but visible from inside the cupboard? And the frame is only 50mm wide, so not a hug amount to screw into?

 

[Amplidyne]

Not pretty maybe, but practical. They don't show outside after all, so you're not always seeing them.
You could fix some vertical battens to screw the frame to inside the cupboard. and then plug the internal holes, and either plug, or as I said fix architrave over the outside ones. or even fix the battens to the front frame with dowels or whatever glued, again screws and plugs inside.