Q about weapon traders and inscriptions

Can you for example put two 20/20 protection inscriptions on a divine staff brought from the weapons trader and make them like some of the Green weapons that are dropping with triple mods on them?

You can only put one inscription on a weapon.

There are other mods as well as inscriptions, which don't require a slot to apply it a weapon, although they correspond to a certain weapon type (Staff heads, Sword pommels etc).

If you look at GuildWiki, you'll find that most Greens can be perfectly replicated for a smaller cost.

The weapons with dual Halves casting time have both an Adept Staff Head an an inscription that has Halves casting time.