Macbook Pro 5,1 will only boot after multiple SMC resets
I’ve seen this before and it could be one of two things (generally speaking):


failing power supply 
failing/failed SMC chip


When I mention "power supply" it can mean either the physical power supply like that in a Mac Pro (workstation) or the circuitry on the logic board of a MacBook (laptop) that manages where the current is sourced from - battery or external adapter.  There's power that goes to the SMC so that it can handle how and where power is distributed throughout the logic board.

It wouldn’t be a battery because when power is removed from an SMC chip, it actually resets.  So, resetting an already reset SMC wouldn’t do anything as you’ve demonstrated with the other numerous resets. However, what we have to remember is that when an SMC reset occurs, the whole system is being reset (warm power cycle). 

The power supply (circuitry) could be failing in that not enough current is getting to the correct power rails until finally there’s enough capacity to get it to work; there could be dry solder contracts on any of the SMC components including the chip itself; or it could just be dying. 

The least expensive DIY test you could do would be to replace the power supply with a known good one (if on a workstation), next followed by a replacement of the logic board.  Unfortunately, to get a proof positive diagnosis, you’d need high level told like an oscilloscope to accurately measure the power going to the individual power rails. 

Ultimately, you'll need to send it in for service because there's nothing more that can be done from a DIY perspective.