oriol

https://noctua.at/en/nh-u12s

Easily! https://ncc.noctua.at/cpus/model/Intel-Core-i7-10700K-416

NH-D15S will still overhang the RAM slots on most boards if it is installed to exhaust to the top with the short side facing the RAM. Go with the NH-U12A if you want to stay clear of the RAM completely. Performance is similar.

You can get the Noctua ones on Amazon, they're called NA-SAV2: https://noctua.at/en/products/accessories/anti-vibration

I'd put it as a top exhaust and use the LNA or PWM control to cap its max. speed to 1200. For the cooler, NF-F12 chromax will be fine but you may also want to try running it with the center fan only. Temps will only be a few degrees worse (at best) and there will be less noise.

Where does it say it would throttle? U9S is fine for the 3900X: https://noctua.at/en/cpu/AMD_Ryzen_9_3900X If you want a bit more headroom with full RAM compatibility from Noctua, take a look at the U12S or U12A. Comparing coolers based on the number of fans is nonsense, this is like assuming that a 4 engine plane is better than a 2 engine plane.

https://noctua.at/en/whats-the-difference-between-nh-d15-and-nh-d15s Perhaps because 2 fans are more expensive than 1. Both will handle the 3950X fine: https://noctua.at/en/cpu/AMD_Ryzen_9_3950X

This may differ from market place to market place, but I've been doing this on Amazon Germany for many years. You can simply select bank account as method of payment.

You can also order on Amazon using a bank account rather than credit card.

Noctua has recently stated that some of their products are out of stock due to high demand and should be back in stock shortly: https://twitter.com/Noctua_at/status/1170943613565624320

iPPC fans don't come with LNAs and LNAs generally don't work with their three phase motors! If you want to slow it down manually, get the NA-FC1. Alternatively, you might also want to try Speedfan.

The B9 redux will work fine, I don't think you'll loose more than a few degrees as compared to the higher speed stock fan. As you already have the B9, I would suggest you simply give it a shot and check the temps.

Go with the U9S, the U9B SE2 is an old model that doesn't even have PWM control yet. Also, the U9S's fan will clear your RAM whereas the U9B's will overhang it.

Better be careful with those numbers, they might seem objective, but they're similarly dubious as airflow or pressure specs with fans: Firstly, those parameters as such don't mean much because because they are abstract and lots of other factors (bond line thickness, mounting pressure, spreading, etc.) come into play in real world usage. Secondly, some of the numbers out there seem highly exaggerated. Noctua has an FAQ on why they don't give these specs: https://noctua.at/en/home-product/nt-h2-3-5g/faq#209

You may also want to take a look at the NF-A12x25 plus NA-SFMA1 adaptors. This actually outperforms the NF-A14 in many applications.