When using Oxalic acid, which usually comes in a dry crystal form, mix it with hot water, as hot as you dare, reading the labels for mix ratios. I mix it a bit stronger than they say, but that is me, I do stuff like that. You are new to this I don't recommend you do that, stick to the directions. While the mix is hot, pour it on the floor the same way the stuff that stained the floor did. Let it set as long as it takes to get into the same places as the fluid did that did the damage. According to the end grain discoloration, it was wet for a long while.

Keep it wet, keep the mix hot if you can, the hotter it is, the easier it works, the faster it will soak into the wood end grain. Do not let it dry out over the time while you are working with it, keep it wet.

If you went really overboard with the acid, I don't think you would ruin the flooring. Yes it will swell a bit, but it will dry back out. Any small gaps created by the flooring swell, will far less noticeable than the gray streaks the stain has. The trick about keeping it wet is, when oxalic acid dries it forms at the least a white haze. Not dry=no haze. If it does haze add more acid, a simple fix. The important thing is the cleanup. Have a clean 5 gal. bucket of water, a scrub brush and plenty of towels. Use the brush with plenty of water, drying it up with the towels. Do this 3 or more times, 5 is not out of the question, cleaning MUST be thorough. I had done this on wood walls with a hose and brush with constant running water and still had haze here and there.

Let the floor dry out for a couple of weeks before trying to repair the finish.

The first thing you need to do is find out where the light is fed from.

fed from a lighting circuit:

Putting a 13A socket on a lighting circuit is not expicilty prohibited. However there are a couple of things to bear in mind.

Lighting circuits are normally on a 6A breaker, plugging in a large appliance my trip the breaker.

Sockets intended for general use with a current rating must be RCD protected. Unless the consumer unit was replaced in the last few years the lighting circuit is unlikely to be RCD protected.

If it's possible to get a cable back to a socket circuit without unreasonable effort then that is what I would do. If not I would fit the socket on the light circuit but clearly label it with something like "burgler alarm only, fed from lighting circuit".

If you do go for the socket circuit option remember to follow the normal rules for that (which depend on exactly what type of socket circuit it is)

fed from a socket circuit via a fused connection unit:

This is similar to above except you will be more likely to have RCD protection in place.

fed from a socket circuit without a fused connection unit:

You need to add a FCU to procide overcurrent protection. Cable from the socket circuit to the FCU should be at least 2.5mm². I would advise fitting a 5A fuse though you may be able to fit a 13A one depending on the type of light fitting and the size of any wiring.