I keep my CPU running at a confortable 50°C to 60°C.

There are 5 fans in my tower, two on the CPU cooler, only one of those two fans is running constantly, at only 200rpm. All the others aren't running most of the time.

I don't need my computer to run at a cool 30°C all the time. The hardware can run very hot without any issues. And when all the fans eventually kick in under load, it will always keep under 70°C anyway.

It does mean things run warmer, but generally moderately specc'ed pcs don't generate enough heat and have enough reliability margin that it is not a problem. If you were some game enthusiast or crypto miner running multiple flagship GPUs on an 850W power supply, then I probably would not recommend this approach.

I put mine in this: https://www.ikea.com/gb/en/products/desks/table-tops-legs/al...

And it's doing fine. Its cooling is slightly over-sized since I want to keep fans spinning at very low speed, but I haven't seen much difference compared to when it was outside.

Yes, depending on how well it seals. Usually there's a hole in the back for HID cables, and the front has rubber feet to prevent door slamming that also offsets the door. This allows heat to flow out.