I think you are OK provided your "transits" through STL are LESS than 24 hours, not more.
The transit through Asia a second time is OK as you describe it - a through transit without stopover between SWP and EUR.
And no problem to break the XONEX with a seperate RT ticket home - that is the way most of us get maximum travel out of our RTW tickets.
Finally, I don't think the 4 EUR sectors you propose violate any rules, though I am not familar with the flight schedules on these routes.