Can Intel refuse to sell such license to nVidia or ask for much higher fees than other companies? If nVidia bought one of those companies, would that allow them to produce x86 chips at will?
Yes. There's no requirement to sell licenses to patents in general. Sometimes (not always) if a patent is included by a government in a standard that government will require the owning company to license it on "Fair, Reasonable, And Non-Discriminatory" terms (FRAND). Most patents, including the x86, x86_64, and various extension patents are not required to be licensed under FRAND terms.
AMD and Intel have a mutual licensing agreement. When Intel patents a new extension, they license it to AMD. Likewise when AMD patents a new extension, the license it to Intel. That's why Intel CPUs use the AMD x86_64 instruction set, AMD CPUs use the Intel AVX 512 vector instructions, etc.
There were some copying and I think they settled with AMD licensing the Intel FSB for $$$ (they switched to DEC Alpha bus protocol when the AMD Athlon 64 bit era came around).
With the AMD 64 they licensed it back to Intel when it became clear that Itanuim was dead as it was so slow and Microsoft forced Intel's hand by releasing a version for Windows Server.
The market is big enough for both.
Yep, but they're both willing to use the patents to keep other entrants out.
It's a poison pill which means Intel themselves would be the only serious buyer.
[1] https://www.trustedreviews.com/news/amd-reveals-its-ready-to...
Which market regulators would never allow. This is even bigger in terms of monopoly than Nvidia trying to buy arm.