Onoff

I'd use Forever White or BT1. Both have fungicide in them.

In 10 years, all good: I did have to back it off at installation and smear with Jet Blue Plus because of a drip:

Soda blasting too: https://premiumdoorstripping.co.uk/what-is-soda-blasting/

How about CO2 paint stripping? For example: https://iceblasters.co.uk/

What will look odd is if a groove is off centre to a single gang switch/socket.

Mice can get through a hole the diameter of a pencil. Any holes you find, stuff with cheap stainless steel dish scourers. For example: https://www.savers.co.uk/household/all-household/cleaning-products/cleaning-sponges-dish-cloths/super-bright-stainless-steel-scourers-6-pack/p/574155

Top tip, never work with cables if you've been eating, sandwiches etc and not washed your hands thoroughly. All that crumby, oily goodness leaves traces on the cables that makes it attractive to rodents. You could invest in a can of Anti Marten spray. I spray vulnerable areas and under the bonnets of parked cars with it.

Freezer on the right, fridge on the left. Leads run from both appliances to a socket behind the cupboard on the left. Somehow SWMBO and I didn't kill each when one was feeding and the other grabbing the leads.

Are you sure, when he pushed the fridge in, he didn't trap / run over the cable with the fridge?

You're messing with us surely? Feed a string through from the right hand cupboard, tie it to the fridge plug before you push the fridge under the counter. Pull the string as you push the fridge in. Sometimes easier with two people. Or get the fridge as close as to the counter to whilst still maintaining arm access to feed the plug around / under the divider into the right hand cupboard. Again one person pushes the fridge whilst the other grabs and pulls the plug end through.

Look for slots that you slip a small flat bladed screwdriver into to open the terminal cover.

Tell her to go shopping if you're trying to get stuff done.

Look at the plug the other end? Gen 2, proprietary plug either end. Gen 3, RJ45 plug either end.

If you must use fast fix boxes, use a multi tool to cut the hole. Then paint the cut hole in the plasterboard with PVA and let dry before fitting the boxes. It keeps the cut plasterboard edges together.

Again, similar but no flats. Unbranded so fits with cheap builders: https://www.toolstation.com/elbow/p46696?