I think what I would want to do is get the server to the minimum to POST, power it up, and see what happens. You should be able to remove the PERC without issue, just not the riser it sits in.  Min to post for this unit is as follows: •System board •Risers•Processor (CPU1) with Heatsink•Power Supply•One stick of memory in dual in-line memory module (DIMM) slot A1•Control Panel with cable Everything else (including the troublesome PERC) can get pulled. You might also look at flashing the iDRAC firmware from iDRAC Settings > Firmware Update, since the iDRAC would work with the BIOS. If it still won't POST, I'd be looking at the system board for replacement. As a point of clarification, when you said that it wouldn't start without the PERC riser, were you removing the PERC PCI card and it's riser? Or were you referring to the PERC as a riser itself? Don't want to recommend anything you'e already done,  after all.  
I know this is an old box, but it was running fine a month ago. I am attempting to use this for a home lab. I have tried minimum boot configuration, full configuration (as it was last running), and everything in between. The system begins to POST, but never goes further than about 20% of the progress bar. What I find interesting is that I can find nothing about a green LED on the PERC 6i card which blinks ~ on 1 second, off 1 second, etc. On the PCB next to this LED is white screenprinting reading "HEARTBEAT_ERROR" and I cannot find anything related to this. The boot process includes the full-rev on fans, which then settle down, the DVD has power, the populated 2.5" drives power up and doe the usual LED light show, then settle to solid green, and nothing further occurs. Removing any of the drives throws an error on the display screen, same for power supplies. It will not power up at all is the PERC riser card is not inserted, or if there is no memory. What else should I look into? Specifically interested in the green LED on the PERC card. 
PowerEdge R710 No Post: PERC green 'Heartbeat_Error' light slowly blinking