Public Windows Phone 7, 8, and Windows Mobile 10 devices were all arm. Intel cancelled further development on Atom for phones at roughly the same time Microsoft announced Windows Mobile 10's desktop convergence feature (plug phone into monitor and get a desktop-like experience, I think there was a wireless option as well). It's pretty simple to speculate this would have been way more nifty on an x86 chip, because Windows Mobile 10 was more or less Windows 10, so you would have a bigger software library, instead of only UWP applications.

Commercially made x86 Android phones exist, the most popular to my knowledge were some of the Asus ZenPhone models.

It existed, but why bother?

When Atom was released in 2008, the A9 had already been announced (a year before). A9 was around 10-15% faster per clock and were often multicore meaning a 1.5GHz chip was faster in all metrics over the 1.6GHz Atom.

A few articles came out June/July of this year with Analysts saying that Intel had spent over 10 Billion dollars trying to break into the mobile market with no success. ARM's current R&D budget (according to Nvidia a month or so ago) is 0.5B. If they spent that much every year since 2000, they would barely match Intel, but their entire market cap in stayed under 4B all the way until 2009. Remember, that R&D includes their high-end ARM cores, but also GPU, midrange designs, various microcontroller designs, a realtime OS, ARM tooling, NPUs, various kernel support, etc

If that much money can't fix up x86 to keep up with a budget a fraction of the size, I take that as proof that the ISA really does matter.

There never were any. Intel cancelled their Broxton phone SoC in 2016: https://www.anandtech.com/show/10288/intel-broxton-sofia-sma... (A major mistake on their part IMO.) There was some speculation back then that Microsoft might have been interested in x86 phones but no proof ever emerged.