1. Yes, it is perfectly normal for the pistons to move back and forth freely. In fact, if they don't, you have a problem!
2. If there IS a trick to this, I haven't discovered it yet.
3. The trick to getting the magazine spring back into its tube is to not take it out in the first place!
Seriously, unless I have a specific reason to clean the magazine tube spring, follower, and tube interior, I don't disassemble it. I certainly don't clean the interior of the mag tube as part of my routine field stripping/cleaning regimen.