After adding the third PSU you're within limits so you don't have to worry about that. However powering your riser with SATA should be a concern. The reason being is that SATA is rated for 55W while your card can be pulling up to 75W. In the worst case scenario your card will be overdrawing power from the riser and that could kill your card or even your whole system. Some users here have had SATA powered risers with no problems but if you have the option to power them via Molex I highly suggest you do so.
For your first question yes this is safe, just tread at your own risk with the risers.
Your second question confuses me. Do you have the option between powering the risers with either SATA or 6 pin? Which version of risers are you using? The riser should have either SATA power, molex or 6-pin. Using 6 pin extenders is ok but that opens you up to more points of failure in your system. The optimal setup would be to power the riser straight from the PSU with either Molex or 6-pin cable.
Third question. Try running the benchmark first with one card plugged in. If that works go on adding more cards one at a time. That helps you isolating your issue when troubleshooting. When all else fails run DDU to remove your graphics cards drivers, re install and try again with one card at a time.
Here are some good resources that helped me when I was starting out. BBT's videos are long but very helpful an packed of information. You can let them run on your second monitor but commiting 1-2 hours of devoted attention to each one can be tough.
Good luck.
https://www.reddit.com/r/EtherMining/wiki/indexhttps://www.youtube.com/user/BitsBeTrippin