This is what I did. You have to have a school email address, my high school email worked, even though it wasn't a .edu domain. Once you get the pack, you can use the digitalocean $50 credit. It requires a credit card(no charge) or PayPal(minimum $5 charge) to use the credit though. Plus, you can get so many other developer tools with it for free, including PHPStorm.
100 mb each player prefered. However depends more on plugins. Also type lscpu in terminal to see your cpu frequency wich is important
There is no metric for this, because there are far too many variables. CPU speed, Bandwidth, type of plugins, number worlds, number of chunks and entities at any time, it's impossible to make a guess that's even close. I will say 91 plugins seems like a bit much, and will likely have an impact assuming some of them are more complex. The second core is really not the best for PMMP, as it's stated to handle single cores really well by comparison. You want to prioritize CPU speed, if anything.
I'm sorry, you seem to be correct about the age restriction. The laws are different in Canada. I'm not sure what another option would be.