This should be fine as long as they are Plus devices. non plus devices really need to be paired near the hub first.
It's not so much the antenna range. This unit is much more powerful than wink. A lot of the issues people see is caused by the fact that Hubitat has to follow the SiLabs SDK in order to maintain certification. Wink finagled a lot of stuff to work and that was not a good thing.
As to schlage, that is an issue wholly caused by schlage. Anything with pre 7,10 firmware tends to be problematic to pair. Schlage is not interested in providing anyone with firmware updates. It got so bad that Hubitat removed Schlage from their "official" compatibility list. Wink on the otherhand had a deal with schlage so they would work with them. Another issue is the whisper that has to be done for security during pairing. Schlage locks should be within a foot or 2 of the hub during pairing.
Honestly as @aaiyar repeaters don't care. That said, it's recommended that you pair all devices WITHOUT security except for Locks and Garage door stuff.
You can expand the subnet and it can communicate across subnets but hub mesh will not work across subnets. I have several subnets and don't have an issue.
This is simply for Hubitat's cloud servers and not necessary for operation of the hub. You can turn off cloud access in your dashboards and what not or block it at the firewall. The cloud access is for your Cloud dashboards (see your local ones when you're out of your network), Remote admin (subscription service but unneeded if you have a vpn) and Cloud backups (subscription service, but uneeded for general backups). Again all this can be blocked and your hub will run just fine.
Yep...
As you are doing things, please take a look at this for some rules of thumb
