completely
No!
"Stable" or "unchanging" could be used instead of "constant" in some contexts.
It is in the taste and that is entirely up to you
a
adjacent
Really anything that is solid and not brittle could probably be used for scissors.
No as they are entirely different.
Could is the past form of can. (Sometimes was/were able to is used instead of could.)
Briefed or enlightened could be used instead of informed. Other words could as well but it depends on the context of the statement.
Swell Cool lovely
Yes, of course.
No. They give an entirely different taste.