Yes, you could certainly do that.
In most situations I would advise against it, because you then need to compile the complete stylesheet each time you run it. But in your architecture you are incurring that cost anyway, so you might as well.
It's possible, but not when running from the command line, which is what you are currently doing. I'll ask O'Neil to comment on what is possible in the latest PHP Saxon/C API.
Yes we expect to issue Saxon-PE/C and Saxon-EE/C at some stage in the future, but they haven't been made available yet, mainly because we need to devise the best way to issue and configure license keys.