1, 2 & 3) You can always try, good ideas are welcome.
2) Modified spells have never been used in the game before. The rules change however. In the end it really depends on the effect you're trying to achieve.
3) Research is always a regent action. An aa can support the research action. I believe non-landed spellcasters can use the research action to learn realm spells, but that's really it.
Adding abilities to an existing structure hasn't been done before, but as I said, good ideas are welcome. You might get a no, but then again you might get a yes. If you get the go-ahead, you'd probably need to research it first, then spend an unknown amount of GB on improving the structure.