Of course density varies with temp but that doesn't mean it really matters. When you have graduations, whether etched into the sidewall, stickers on a level gauge or lines on a story board, you aren't accounting for a change based on temperature and those all work great.
Everything is really...
I don't understand why you wouldn't just use weight. I don't think I've ever seen a thread on measuring wort volume by weight. I've seen floats before, level sensors (which may just be floats I guess), pressure sensors. What am I missing?
I think you're fine and you just need to wait a bit longer. How long have they been in the fridge?
I'd put em in my basement by a drain and leave em alone for a couple of weeks. A garage this time of year would be good too. Just somewhere cool that will be easy to clean up if they do blow.
Conditioning in kegs work great. Bottle conditioning isn't really a good name for carbing with priming sugar. Conditioning is aging and carbing is really a separate process that happens to go on at the same time when bottling. You've got yeast in there either way and they'll continue to clean...
Have you tried a worm clamp on it? I bet itll work fine.
Tube is sized by OD and thickness. Sounds like your 3/8 in tube is really 3/8 OD but your 1/2 in tube is really 3/4 in with 1/8 in wall and 1/2 in ID.