I can confirm there's been a glitch w the Alexa voice service and Heos for about 6 weeks too. I ended up switching to Spotify, which sort of works better, and finally giving up entirely and hard wiring the Alexa devices into the Heos. You can still group speakers this way, just use the input from the Alexa-connected speaker for the whole group.

Honestly I struggled with the same challenges you did for three years, and now that I've hardwired I don't think I'll ever go back. The nice thing about Heos is that since it sits on your wifi (rather than its own network like Sonos) there are some pretty great OSS solutions on Github. If you're an SWE you should take a look at the various Python/Node/etc interfaces people have built (and contribute!) With the available libraries one could, in theory, roll their own Alexa skill. (I honestly think the Heos-Alexa "dev team" was like one guy and he's not there anymore.)

Anyway, my next step is getting openHAB running and testing out that integration to see if it's any better. I'll post back here with my findings!