Yes, crazy prices now, it's just not the time to buy. In a couple of weeks, or a month at most there must be another supply of cards (or even new models) so I expect there will be more value for the money.

If you undervolt the cards (and you should!) there is still plenty of space in that 850w power supply. You can add a 5-th card for sure even if you do dual mining. Well, you don't say the models but it's safe to assume anyway.

Previous generation cards draw more power, and they could still be available second hand. They are cheaper than current generation, obviously.

That's a very wrong conclusion. GPU's a very independent. You probably had a problem with correct power supplying to GPUs.

You more or less answered your own question. When it comes to hash power it doesn't matter at all whether you have divide your cards to more rigs.
It's usually easier to set up a 4 gpu rig, less issues, they somehow solve themselves, cheaper power supplies and a better choice. The downside is more CPUs, motherboards, RAMs, hard drives etc.  - it can add up! On the other side, testing anything on a 13 gpu is expensive, anything goes wrong with a 13 gpu rig - you lose a lot of money. If I would start all over now, I would get 8 gpu motherboards. It seems to be a good number where you still have enough gpus there to save money on hardware, you still can get a decent power supply solution and you still don't get to many other issues.

As of power supply, I use all kinds: regular, "mining" and server. I've managed to make everything work. If I would start over, I'd get only server supplies for gpus with some regular pu for the motherboard, cpu and hard drive. Mining ones are a new thing, my guess is they are not properly tested and just an extension of the regular pus. Server ones are cheaper, quieter, have all kinds of protections in them, and best of all, they are built for 24/7 operations by serious companies.

I don't think there is any difference worth worrying about the add2psu and other solutions. I've used all of them, now I prefer anything with less electronics, I just find something to connect the pins (properly!).