Starting problems
Let's get this to the most basic components
If I understand, it won't crank, even when jumpering across the starter solenoid?
Most basic you have block---ground cable --battery -- positive cable -- starter relay "big stud"--down to starter--and through the starter to block, a complete circle
You should be able to short the two terminals AT the starter and it should crank
If not, clip your multimeter to the block and to the large starter terminal AT the starter
Check voltage while jumpering the solenoid
If voltage is below 10, battery is low, cables are bad, or starter is bad
Check voltage at battery posts while jumpering starter. This will take a clip on remote switch, or two people
Below 10, battery low or defective.
If battery is OK, but low at starter, check your battery cable terminals.