So would you say that (in your opinion) a source available license could in theory call itself open source, if it used language you found to be unambiguous about your rights? Or is that not possible?

If it calls itself "open source" and I go to https://opensource.org/licenses and can't find an approved copy of the license - or I can't ask my favourite open source expert lawyer for their advice on it - then I don't want it to call itself open source.