Default [Q] Autostarts and Receivers

I am trying to reduce my phone boot time and boost performance and battery life and want to better understand how applications manage to load themselves on system boot. I have a lot of apps installed, several hundred; and using the Bootmanager in XPosed as well as the Autostarts App I seemed to be able to turn off most auto running apps, however,

1) Some apps still manage to load and I have not a clue how .. so I will turn the phone on and as soon as the launcher appears go to the settings and apps and see what is running, and processes will appear which I have no desire to be there. For example RocketPlayer manages to load itself even though not a startup app and has no widgets etc. It does have several receivers still active but none of these seem obviously related to system boot.

2) If I look at all the applications that have a receiver hooked up to 'Connectivity Changed' for example .. do they all get run when the connectivity changes, or are they only notified if that application is open at the time?

3) in general is it a bad idea to have lots of apps and better to stick to the minimum of what you use regularly?

4) should one just Greenify all apps not regularly used or essential?

Any help on this subject would be appreciated!