Turning on RCSI on on-prem CRM 2015
It's critical that you do not enable RCSI on a database without the application vendor and your own developers certifying that they support it. The reason is that the behaviour of all your read queries will change, but not actually break in such a way that replaying your workload will detect. Read section 3 at https://www.brentozar.com/archive/2013/01/implementing-snapshot-or-read-committed-snapshot-isolation-in-sql-server-a-guide/.

To answer your specific questions: 


Switching on will be near-instantaneous (assuming only one DB connection).
Yes, switching off is just as painless.
See above.
Cleanup happens by itself. Section 2 of the post above has some things to consider.