In reading your post I feel like you have a misunderstanding of HSM and HE modes. They are two separate things. Could you have a change in mode result in a change in HSM? Sure, but you would need to specify that in a rule or app. Could you have a change in HSM result in a change of mode? Sure, but you would need to specify that in a rule or app. Could you have lights turn off/on when mode changes? Sure, just make the automation in one of the various apps that come with HE (RM, SA). Could you have presence set your modes? Sure, but again you need to specify that in a rule or app.
Could you use HE modes and never install HSM. Yep, I am sure there are a few users who do this.
Could you use HSM and never install Mode Manager or use modes in any way. Yep.
There is no right or wrong way of doing this and different users have different preferences. If you do end up using HSM and Mode Manager, it is good to decide early on how you want it all to work together and which one takes preference. By "takes preference" I mean which one controls the other. For example, in my home, setting HSM to "arm-away" results in my mode becoming away and my mode will stay in away until HSM is disarmed. I often have family staying at my house while my wife and I are at work and this means that all my automations work regardless of my and my wife's presence. This works well for me but I am sure other users do it differently.
For me, my home will cycle modes based on time of day (morning, day, evening, night). The only way the mode can be changed to "away" is for someone to set the alarm. This could be me, my wife, or my guests. My presence and my wife's presence has no effect on the mode. I do use our presence, just not for changing the mode.
You can make as many virtual switches as you want, I believe these can be linked to google home (I no longer use, google home so I am not 100% sure, but I am fairly sure). Make a virtual switch, name it "Away Mode" make a simple rule that when switch turns on, away mode is activated. Tell Google to turn on "Away Mode".
Again, there is no right or wrong way to do this, it all depends on how you want it to act when everything is set-up. When, I initially set it up, I had everything automated based on our presence. For my wife and I, this was a pain whenever we had guests staying at our house when we were not home. Plus we always push the arm-away button to arm the alarm when we are leaving the house, and enter the pin on the door lock when we arrive. For us, it made way more sense for it to be set up like we have it now. But like I say, everyone will be different and want it to work differently. The problem I see on this forum is usually when it has not been logically well thought out and you have: presence, mode manager, and HSM conflicting with each other. This is not because these apps naturally conflict with each other, on the contrary they work great together. But you can definitely end up with rules/settings that make them conflict. lol
Yes I believe you can do this. I do the same thing with Alexa. I have an Alexa routine called Goodbye and when I tell Alexa Goodbye I have the routine turn on power to a HE Virtual switch. This switch runs a HE Rule called goodbye. I have the rule below. I think you can do the same thing with google but I don't know for sure. I have other rules for Goodnight, I'm Back etc.
That's exactly what I do. Well, to be precise, what I do is arm and disarm HSM automatically using Rule Machine (RM) based on combined presence rules. So a short while after we go out, HSM is armed. And when we get near home, it is disarmed.
Tell google home to set home, away and night modes in HSM?
Actually I haven't done this but I think it should be possible using a virtual switch inside GH and a few GH Routines. But I never bother. You can set modes automatically based on anything else you like (such as time, lux, presence, etc). So why use voice? I like to automate my house.
When I’m in away mode automatically turn off all lights.
Easy peazy.
Set a schedule for night mode.
For sure. Just use RM to set any mode based on time or anything else you like.
As general advice on this subject, I use Day, Evening and Night modes. Day and Evening are triggered by the average of 2 lux sensors. It works perfectly and is used to control various aspects of my motion lighting. Night is triggered at 11pm. You could set that for when you sleep instead. Or use a switch, button etc to set it. That's it. I don't mix time modes with presence modes otherwise you will end up having many combinations (night-home, night-away etc). I prefer to use a separate virtual switch for presence.