You could use port forwards but I recommend vlans or vpns. Ideally use something like a draytek router and link all sites together via vpn.
Alternatively you could look at smoothwall units or openvpn
This is a IT solution as I wouldn't recommend passing security information over your wan with out strong encrytion.
Any of the premiers could do that but u will need a few modules. Have a look at the Honeywell flex this will do as you need and has app support. I wouldn't recommend sms alerts unless using a gsm module.
The active beams come with pole mount brackets so you can concrete in the pole and attach. No you cannot get mirrors designed for the purpose, while the theory is sound it would be a difficult to setup and maintain.