Contact MMR support to investigate
without knowing what settings renters used, where they pointed the rig, or what coins they tried, it's impossible to diagnose. Problems might be from bad pool configs or connection issues.
Possible, but most of the time I've heard some sellers who do sell cheap hashrate aren't giving you the right hashrate. Like I said before, there are some fraudulent sellers in MRR; they are mostly selling cheap hashrate.
You can't easily find those frauds, but you can rent a small hashrate for testing in a few days; if you notice they are stealing hashrate, report them and move to other sellers.