There're no users who lend you money but instead it was the system itself.
so you do not need to worry about their money because if you are winning it,the system will credit your account instantly.
(from my perspective they are paying you with their own money,because some people win it and at the same time some people lossing it).
for poloniex you can read it here;
https://poloniex.com/support/aboutMarginTrading/thank you
what does it happen if the number of winners is more than the number of losers?
Poloniex in Bankcorrupt?
for your first question,
their profit should be higher than their expense.
because most people lost their money(get liquidated) while the majority people got profit around 20%+ from their capital.
but if that thing really happens,they will run away with it.
(it happened many times before,but most of them ran away with other people money instead of those).
second,i do not really understand what is your question.
but,i do not recommend you doing the trade on poloniex,
they were doing a shady thing,like disabled account and many more.
