Will Seattle ever see single digits again?

1. The climate isn't warming

2. Even the researchers who believe in anthropogenic climate change claim that there will be colder extremes alongside the warmer extremes.

The hell if I know. Probably not, but OTH Seattle is just south of a very cold continental landmass.

No

Only happened once since 1894, and with Seattle already becoming warmer due to climate change and UHI it’s highly unlikely.

Outlying areas might if there’s an extreme cold snap but not in the UHI.

Likely gone the same way as -20 f here.

We've gotten down to the single digits more than once. www.extremeweatherwatch.com/cities/seattle/lowest-temperatures-by-year

It depends where in Seattle. The city proper and near the water, likely not. I'd say less dense areas (less UHI) farther from water could do it though.

Vancouver has seen -13.7C/7.3F this January, and Seattle ain't that much different from Vancouver I guess, so yes Seattle should/might see single digits in the future.
Also, the word "ever" implies a big ass future time period, and I wouldn't jump so quickly on ruling out some temperature threshold.

I mean, even if the climate is indeed changing, Seattle will still definitely experience single digit temperatures, at least one million years from now long after humans have gone extinct, the anthropogenic greenhouse gases would have already left the atmosphere and Seattle would return to its default climate. Not to mention the ice ages yet to come, which will make the location that used to be Seattle will commonly experience constant single digit temperatures during the winter.