How feasible would a large scale, state sponsored glass industry be for the North (panes of glass for greenhouses) and do you think that most large castles and towns in the North have glass gardens?

No, Winterfell seems pretty damn unique when it comes to the glass gardens.

The main issue in glassblowing is the availability of quality materials and skilled labor. You need large quantities of sand (preferably fine sand), fuel (because you need to heat up sand quite a bit to get glass), and something to act as a flux (an ingredient to lower the melting point of sand, such as natron or other salts), and the North’s western beaches seem to be largely stone and pebble. So maybe on the east coast?

And then, as I’ve said before, you need to import skilled workers from Essos.