Electrical – plug a driver/transformer for 12v undercabinet lighting into the same outlet the OTR microwave is plugged into

This is a 20amp outlet, but before you go drawing 16amps/80% of that, you should verify that there is a 20amp breaker/fuse on the other end as well as 12 gauge wiring (thicker than the standard 14 gauge you find on most 15amp circuits). If there is only a 15amp breaker, then the max you can draw from that outlet is 12amp (80% of 15amps).

You mentioned your one device pulls 8amps - that leaves at minimum, 4 amps available for your freezer - what is the current draw of that device? If its 4 or under and the breaker is 15amps then you are OK. If it is over 4 then you have to ensure you have a 20amp breaker in which case the max you'd want to pull is about 16amps combined.

If it is a 20amp breaker, then you will need to find a power bar that is rated for 20amp - I think most of the time they are rated for 15 but if you look hard enough you will probably find one for 20. You could also replace that one outlet with a 2 recepticle outlet which would eliminate the need for a power bar.

No.

There are a few issues here. While what you're doing could in theory safe, the rules are designed so that "what if"s are taken care of. ie. you're installing a socket which could be used for anything, so you have to allow for that.

1. Sockets now require RCD protection, which you won't have on your lighting circuit.

2. The rating for the lighting circuit will be lower than a socket circuit.

3. You're required to terminate a lighting circuit in a suitable fixture. The rules allow for "a suitable power socket", but the implication here is that you'll be using a type D socket, as is typically used for things like table-lamps/uplighters on lighting circuits. Adding a type D plug to your appliance would probably invalidate it's certification.

4. AFAIK you'll be required to submit a "Minor Electrical Installation Works Certificate" to local building control. While this can be done by a homeowner, it requires you to be competent, comply completely with wiring regulations and have tested the works properly.

5. Your insurance company might require that works be carried out by a qualified and suitably registered electrician.

But! But!
Ok, so for point #1 you could add an RCD socket.
For point #2 you could in theory add a FCU (fused connection unit) with a 3A fuse - but here you run the risk of someone coming along in the future and changing the fuse for something higher. This in effect rules out this potion
Points #3-5 I don't see a way around.

All-in-all - this has bad idea written all over it.