There are two engine grounds. One on the right front head cover
The other is on the middle / left side of the engine compartment and connects to the transmission housing. This diagram of the transmission shows the transmission ground cable. You need to look at the diagram really closely to see the actual cable - it is definitely not the clutch slave cylinder mounting bolts.
Based upon the diagram, I am thinking that you might need to loosen or remove the air box and the control box to get eyes on the ground strap mounting point - I will defer to somebody who has actually had to do this. I am thinking that your "former NSX tech in the Orlando area" had an oops moment and missed reconnection of the ground cable during the clutch replacement. I leave it to you as to whether you want to take up the error with the former tech.
Have a look at the second ground strap at the right front of the head. My car is in storage so I can't check it; but, my fuzzy memory is that it is a smaller strap and may not be rated for the starter motor current which can approach 100 amps. Check the complete length to see whether it has been damaged by overheating. Heat damage will be most likely at the crimped terminals on either end. When you crank the starter motor with the main ground strap disconnected the starter current flows through the smaller ground strap which can cause a significant voltage drop between the engine block and the rest of the car resulting is erratic operation - as you seem to have discovered.
I did a clutch hydraulic system flush last year. I did not pay a lot of attention to the mounting bolts for the slave; but, my fuzzy recollection is that they are not big bolts (M5 - M6???). If that is the case I would not use them as a temporary grounding point. If the bolts are in fact larger diameter and the ground lug does not flop around on the bolt, then they may suffice as a temporary ground strap mounting point.
The starter motor circuit is pretty simple
Battery to ignition switch to starter cut relay to the starter motor solenoid. Once you get the engine ground cable sorted out and presuming the cables back to the battery are OK (the battery post cable clamps and the battery ground cable are common failure candidates) the likely problem candidates are
- the ignition switch - which you should have eliminated
- the starter cut relay which includes the security system and the clutch interlock switch (I assume you have checked to confirm that the clutch switch stopper has not dropped out)
- the actual starter motor
After the ground cable problem has been fixed, the fastest way to isolate the problem area is a jump test on the starter motor.
- get about 1m of #14 wire with about 1cm of insulation stripped from each end for use as a jumper.
- remove the blk/wht starter wire from the starter motor solenoid
- make sure that everything is clear of moving stuff on the engine because the next step should cause the engine to start turning. Make sure the key is not in the ignition switch because the engine can start if the key is switched to run.
- carefully hold one end of jumper wire on the jump start terminal in the engine relay box and then touch the other end to the terminal on the starter solenoid. This should cause the starter motor to engage and turn the engine over.
This test by-passes everything in the starting system control circuit. If the primary (big cables) connections are OK then you likely have a starter motor problem if the motor does not operate. If that is the case, chances are that the bad ground connection may have resulted in a low pull in current on the solenoid leading to burned contacts on the solenoid. After removal of the starter motor it may be possible to clean up the contacts with a file or there are contact replacement kits available for less than $20. If you search on Prime there is a thread from about 5-10 years ago describing replacement of the starter solenoid contacts. You may also be able to find a local electrical specialist who will rebuild it for you. Remans are available; but, the quality can be dodgy.
If the starter motor operates that pretty much narrows it down to a bad electrical connection somewhere in the starter circuit or a problem with the starter cut relay circuit which is a headache that likely requires a completely separate thread