It looks like a neaty arranged plate of samosa to me!

I don't think coir can really be used as a straight swap for peat.
I've used ericaceous peat-free compost for peat- they're typically a mix of coir and bark products.

I still use a JI based compost with about 1/3 drainage. I'm not growing plants in a greenhouse so flooding pots when watering is something I need to avoid. So I'm not too keen on mineral composts. Getting a good JI substitute that is peat free is still a challenge though. Biggest step forward for me...

Even the pigeons don't seem happy with today's weather...

There is Hypertufa -made out of cement, sand and peat. Of course you'd might have to swap the peat for something similar.
I remember making containers out of this years ago- over time the cement component settles down and does not leach too much.

There are quite a few You Tube videos out there where people use angle grinders and the like to make containers
out of natural stone. A different approach to consider for making stone containers, as long as you have the kit and
safety gear and can control all the dust that you'd generate!

I've seen the odd tiny mealy adult as well. I assumed this was that they were just under stress.

It does seem to be one or other of the true Rebutia that flower first. I've got a R. minuscula indoors that is flowering now. The rest generally follow on a couple of weeks later. I grow these plants where they only get modest amounts of sun- they flower well but spination does not look anything lik...

That's quite a display you are getting indoors.
Do you give a cold rest or stop watering over the winter to trigger flowering?

I let the temperature drop ~10oC at night, so it goes much lower than 15oC at night.
I have my seedlings indoors so this is relatively easy to achieve.