Scotts S1642 no start, low voltage condition

I have a Scotts S1642 made by John Deere, and I have a no start condition that I'm not sure how to proceed to diagnose and fix, so I'm reaching out to the community for ideas. I've been attempting to diagnose the

problem for some weeks with no success.

The mower has been working fine, with regular maintenance done. I went to use it, and the mower did not start. I am getting the fuel solenoid click, but starter will not engage. My first thought was to check the

battery, but the battery showed good at 12.8 volts. I checked the voltage at the battery cable attached to the starter, and the starter motor was only receiving 5-7 volts. Over the next couple weeks, and what is now a

series of misdiagnosis and my own self-education on lawn mower electrical system, I am still at a no-start condition.

I did a complete deck rebuild in October of 2014, and the motor and transmission run great. I rebuilt the entire deck, replacing everything except the deck itself, and while the deck was off, went so far as to replace the

transmission drive belt, tension spring, and all the under frame idler pulleys. Rebuilt the carb when I replaced a faulty fuel solenoid. I wanted to make sure that this was the one and only time I was going to have to

perform a job this involved with this mower. When I don't run into situations like this, the mower runs great for being a 2001 model. I don't feel I'm ready to give up on this mower for what may turn out to be a simple

fix.

Before I just started replacing parts I had done the following, with no success, which lead me to start ordering what parts I thought may be faulty:

tried jumping the solenoid, no start, replaced solenoid
tried jumping the starter, no start, replaced the starter
replaced ignition coil, because I didn't know where else to check at this time and was getting frustrated, and thought that by throwing another $70 at it, it would somehow magically start right up
replaced ignition switch, thought maybe not letting complete 12 volts thru, still no start
had battery load tested, passed, still no start
thought maybe one or more safety switches not letting 12 volts thru to solenoid
checked continuity on all switches, passed
something is still not letting 12 volts pass thru something else to get where it needs to be

Battery check from post to post shows 12.8 volts.
Battery check with positive post and ground to frame shows 12.8 volts.
Battery check with positive on positive post and negative to frame, try to start, voltmeter shows drop to 5-7 volts.

All cable/wiring connections are tight and clean.

I've replaced the following with new parts:
starter mower
starter solenoid
ignition switch
positive battery cable
ignition coil

I've tested the following:
battery under load, passed
(battery is less than one year old and I top off the charge with 2amp slow frequently)

continuity checks done and passed on:
PTO switch (under deck left side)
brake safety switch (under deck right side)
seat safety switch

I've gone over the wiring harness to check for breaks or loose grounds, and found none. All wiring harness connectors have been checked and cleaned, ground points are tight.

So I've done all this, and I still have the same problem. The mower won't start, and it seems everywhere downstream of the battery where I should be getting a reading of 12v, when I try to start, I read 5 to 7 volts.

I've been trying to Google this issue for a couple weeks, and everything I seem to find seems to diagnose a problem where 12 volts that gets to a point, then no voltage after. I can't seem to find anything that references

a "not enough voltage" condition like I describe.

I need ideas on what else I can look for that is causing this condition.