Glass is hardcoded, so yes, it is magmaproof.
Farmplots cannot become "unseeded", but if you plant a plant in autumn and it cant grow in winter, and before the plant is fully grown winter engulfs you... then the plant dies, and the seed is lost.
Example: You plant surface plant, which needs 1 season to grow in spring. It is active in spring, summer and autumn. It will be fully grown in summer, and you harvest it. All good. If you plant the same plant in autumn, it would be fully grown in winter, but it does not have the active_season:winter tag, and will die.
All underground plants have all seasons active, all aboveground plants have all seasons but winter active. Only plant them in spring/summer though, otherwise you lose them.
The flora&fauna plants, which are the "more plants" in the GUI, I did not write and cant say anything about. I assume they also dont grow in winter.