Will we be reliant on our cell carriers to push the updates or can we get them direct from Microsoft over wifi or cell?
All updates for all Windows 10 devices around the globe, including in the US, will be ultimately controlled by Microsoft.
I trust my Lumia 640's firmware and OS, like any Win 10 supported device, will have the same level of support as newer models which have or will come out, no different really from pc's. It's reassuring.
That's the whole idea. One OS to rule them all. To make it One OS, it has to be able to be kept up to date.
No different from PCs... yeah, that's on purpose, too. Microsoft is doing it right this time around. They may be taking their sweet time to get it there, but it takes time to get it done right.