Even if you don’t have a C wire, you can use an external power brick to power the smart thermostat in place of a C wire. Otherwise at least the Honeywell thermostats come with a C-wire adapter than can be wired into your furnace control box and shifts some of the wires around to provide a C wire. I found the external power brick to be a little less daunting, but either option should work for you.
The Meross is just a relay with wifi, so you would have to figure out whether you can close/open the door through one of the terminals. The terminals on the left are the high-voltage side, so I would not touch those.
You could try putting a jumper between terminals 19 and 20 which is the push-button connection. If the door opens or closes then you just attach the two wires from the Meross device to those terminals. If that does not work, you may need an attachment to the Meross unit or something else entirely.
Here is the wiring diagram for that opener - https://irp.cdn-website.com/cff40d41/files/uploaded/RD20X2TELL.pdf You should be safe connecting terminals 19 and 20 since those are low-voltage connections, but if you are unsure maybe talk to someone who knows more about those specific garage door openers.
Yes, that’s certainly possible - you would just create an automation in Home Assistant that toggled the garage door when the switch is pressed. A switch like that is designed to replace to replace a regular light switch, so when you trigger the switch it’s also going to trigger whatever that switch is wired to. You want to make sure that switch works with Home Assistant - most do, but if you can get something that runs Tasmota or ESPHome then you will know you have something that works with HA and is fully local rather than depending on a dodgy cloud service.
If you want something that operates independently of the light switch, there are a bunch of Zigbee battery-powered switches that work well if you have a Zigbee dongle on your HA box. I have had good luck with these if you use the Home Assistant blueprint for them: https://www.amazon.com/Aqara-WXKG11LM-Switch-Wireless-Remote/dp/B07D19YXND/ref=sr_1_12?crid=31P95EAUGD369&keywords=home+assistant+scene+controller&qid=1699888273&sprefix=home+assistant+scene+controller%2Caps%2C97&sr=8-12