Could viral or other infections cause puss pockets in the back of throat, besides strep?

What you consider pus pockets is often just injured/shedding throat tissue. One of the worst looking throat infections is caused by mono. There are many other adeno and enteroviruses that cause nasty looking throats. If a strep test is negative, we assume it is viral since there is no value in specifying which one other than for research.

Definitely so, strep is well described and easily to diagnose due to availability of tests, almost any other virus or bacteria can cause such changes, for bacterial identification, a throat culture may help and specific viruses can be identified on viral panels, most commonly and well described EBV for mono.