Most of the affordable cell backups are specifically designed to deal with standard CS signaling. Telguard does manufacture some (very expensive) cell backups that will work on an analog cell network on the voice channels and provide functionally the same srevices as a landline. The equipment is expensive, and the service is expensive.
If you buy your equipment outright you can probably have it monitored including cell service for about the same amount (or less) using your landline for your primary communications and a modern contral channel unit for your cell backup. If you want to save on the cost of a landline (not have one at all) and use your cell for all communications the cell service will be slightly more, but the difference will probably be less than the cost of the landline. This is using a modern cell backup that uses the data control chanel of cellular.
I'll leave it to others to tear up the rest of your post.