Multiple hubs, yes. You can select devices to be "mirrored" across to another hub on the same LAN using a core function of Hubitat (which has just been significantly sumplified/improved). There is also a long-standing community app that can do this and furthermore ecan do it across hubs in different locations/LANs.
Yes, you can set up multiple users and select their mobile device to send a push notification via the app.
Not sure about the last question. I've only ever set up one alarm/HSM code. It should be possible via Rule Machine even if not a core capability of the hub. You could have a dashboard tile which allows entry of a number or string. This can be linked to a variable. Then Rule Machine could check that variable number when it is changed/entered and disarm HSM based on if that code matched one built into the rule. That should certainly work OK. Maybe someone else can comment if there is a better way.
Regarding other functionality, check online docs...
I dunno maybe @danabw can chime in - he's part of the cult of Apple I think..
I am not using the HE DB as my setup is more oriented toward automation than remote control. I do have a dashboard that controls my HE devices via a separate system called Node-RED that I can access anywhere via a locally running VPN (WireGuard). The NR DB is more rudimentary than HE's but for my use-case it's fine.
I just recently made the transition, but for reliability reasons. It seemed ST had some sort of system-wide issue weekly, and I had little local issues all the time.
Have you tried webcore on ST? It also exists on HE, and it runs locally instead of on someone else's computer.
My HE doesn't seem to let me add additional users at all. However, you can assign dashboards to people to limit what they can control. The dashboards can either be in the cloud or local, from the looks of it.
This may still be the case, but dashboards sure are customizable. You can access them inside the app, or with a only a web browser.
I like it so far. I ran into a few devices that don't work with HE, but did with ST. I'd check the compatibility list, or search elsewhere for drivers to prep. I also ended up purchasing a z-wave stick to include some S0 devices without security to increase performance and reduce issues, which was recommended by another user.
There's an app for that; I haven't tried it. I just transitioned one or two rooms at a time.
It looks like I can, but I only have one device attached at the moment. You can for sure do this with pushover, if you can use the native app in this way.
I like that anything that can be done on the phone can be done on the PC. I prefer operating from a PC over a phone, except when pairing devices that aren't portable. The response time for local stuff is crazy fast.
For me, it's slower for me than using a web browser to control HE. We are in agreement; the app speed doesn't seem to be an issue in ST. For me, adding a scene shortcut on the android launcher reduced any speed concern (so you don't have to open the apps at all).
My list of cons (when compared to ST):
Crashing z-wave radio (resolved using advice from other members and support)
Less compatible devices (most missed: Ikea buttons)
Some apps cause performance issues, when they didn't in ST cloud (most missed: influx db logger)
No wifi (not a big deal for me)
Weaker signal strength (just add a plug nearby)
To be fair, my con list for ST:
Musical chairs with device control: The old app, new app, and portal (not one does everything you need)
Cloud controlled stuff
Vanishing features (Echo speaks)
Non-stop maintenance and changes
No nest support (got SDI beta working on HE, it's awesome, but difficult to setup)
Overall, I'm happy with my purchase. The grass wasn't always green during the transition, so plan on a little transition woes.
When I moved from ST to HE I used HubConnect to ease the transition. HubConnect allows you to control devices paired to your ST hub from your HE hub. As I built my HE automations I used HE groups instead of individual devices. That way when I moved devices from ST to HE I just added them to the group and I didn't have to mess around with a whole bunch of automations.
Many folks (me included) do not add zigbee bulbs directly to HE but rather use a secondary hub (in my case Phillips) on a different channel to control zigbee bulbs. Zigbee bulbs tend to cause havoc because they will repeat ZLL OK but don't do a great job on ZHA. There's really nice integration between HE and Hue.
Here's one of my dashboards...taken from a browser not a mobile device but basically the same thing. I'm colorblind so mine tend to be less colorful than others