1) Yes. When ever you are not playing you should disengage any levers.
2) That is up to you. The pros for leaving it in Eb is that it will simply be easier when you actually want to play that one song. Also if you leave it in Eb rather than retuning it will keep the tuning more stable. The cons are that engaging the levers does tend to cause a bit more wear on the strings. Also depending on the levers you have some harps you can notice a compromise in tone when the levers are engaged so no need to engage them if you don’t have to.
It’s a trade off, but keep in mind, as you play more and get better you will almost certainly come across more music that will require Eb. It may be a good idea to get used to having your harp tuned that way (and let your harp get used to being tuned that way) in expectation of a broader repertoire in the future.