Aiwa

Preview of what's coming up soon  ( @RottenX also includes your suggestion #1 ).
 

I don't know if you know the site https://gumroad.com/, but I'm going to use it as an example of what I would like to have as an option for files in the suite.
In that site you are allowed to specify a minimum purchase price for your products. So lets say that I put something for sale at a minimum price of $5, people can buy it for that amount, or they can pay more if they wish.
Now, what's more interesting here is that you can set free files, setting the minimum price to 0, but people can still choose to give you some money for it, kind of like a tip.
This is one of my products:

People can input 0 in the box and get the file right away, or they can input whatever amount they feel like.
I'm not gonna reveal any numbers, but this system kinda works, people tips more than I expected for free files.

I can nearly guarantee a plugin that simply selects from the user table to determine who has been active in the past X hours to manipulate the list is going to be more reliable than hacking code, changing server configs, etc.  I'm not sure you understand what you're actually doing here - you're not setting a display cutoff, you're altering the PHP garbage collection config. 
We've had issues with people tinkering with this and I feel compelled to note, you're "voiding your warranty" so-to-speak and we will not provide tech support should you end up with "people aren't staying logged in" etc. Proceed at your own risk.