Most of your questions can be answered here:
https://en.wikipedia.org/wiki/Proto-Indo-European_phonology
Or maybe a little simpler here:
https://en.wiktionary.org/wiki/Wiktionary:About_Proto-Indo-European

The superscript w is a diacritic indicating that the consonant is labial (in this case, labiovelar). In other words, it's the "qu" sound in English, rather than a plain k.

The circle indicates a syllabic consonant (like English rhythM).

I'm not sure about the vowel, which looks like it is marking it as BOTH a short and a long vowel, according to general uses of those diacritic marks. It also cites Pokorny, whose work you can read here (in German):
https://archive.org/stream/indogermanisches02pokouoft#page/690/mode/2up
Doesn't look like there's any explanation of the orthography there, though, or in the first volume:
https://archive.org/details/indogermanisches01pokouoft

One possibility (just a guess) is that it might mean that it is EITHER short or long (either unknown, or that it varied?) -- maybe because different reflexes are found in different modern branches so it could be either one? That might be entirely wrong though.