To answer your first question, 5.8 and 5.10 represent 2 disparate development streams with individual baselined contents and release roadmap i.e. upgrade & maintenance releases will be made from each stream independently and since both are still live, you cannot infer that 5.8.9 was released before 5.10 (or even 5.10.x), nor can you infer the likely contents of the releases relative to each other...

I have no doubt that other, better informed, monks will be able to provide further details e.g. links etc., wrt your 1st question </ignorance>

In (an attempt to) answer your 2nd question, have you tried just simply force upgrade ? AFAICT i.e. I've never tried it, I'd have thought that should force an update to all out-of-date packages.