Thanks for the details, this helps a lot! 
For me the use of Aqara app would be limited to Alarm alerts. I just want those to work in case if HA hangs or an integration falls off. Though now that I’m reading this, I am starting to realise that I might be trying to do something silly. When Hub is connected to HomeKit, it would actually make sense for the app to suppress alarms as they will duplicate Home app’s alarms. I had it previously with Eufy alarm system, (where the integration with HA was through a cloud integration so the app wouldn’t know it was connected to HomeKit) and whele alerts were redundant just how I wanted it was also quite annoying
Sounds like I should be able to get it working acceptably so just need to try it out in different ways. I’ll share the results some time later. Hopefully when Matter is supported it will be even easier. Thanks again!