Concerning 3) I haven't had a look yet myself at the LPM in 2.5.4. Given though that the Active Sessions method used for the module is different from the Administration model and rather specific to the LPM (ie, it sets the last time a user was online and the active sessions grid is constrained using this value) the method hasn't changed. I'll have a look though if anything in 2.5.4 broke this.
Update: I just ran my own LPM project (which is just the LPM module itself) in 2.5.4, and Active Sessions for me worked without any problems for both Server Admin and Pool Admin roles. I also checked the test project you submitted on your ticket a few weeks ago, and Active Sessions also worked normally for that one. As a result I suspect it's one of the following:
On a related note: as you requested, from now on the Mendix Windows Service is released via the release mechanism in the Support Portal.
Regarding 1) - this is part of the monitoring interface on the Mendix Runtime. We use it in our cloud environment. We will create documentation soon to explain the interface and usage if you're not running on the Mendix Cloud Platform.
Yes, send an email to email@example.com
Concerning 4) If those release notes do not mention a reason why this changed it's probably a regression bug... :|