Glow plug replaced, but still won't start

I'm having some problems... Here's how it went down:

• My car was cranking, but wouldn't start, and I was getting glow plug errors in cylinder #2 (P13D1 & P13D3)
• To troubleshot, I tried swapping the cylinder #2 glowplug, with the one in cylinder #4.
• Now, surprisingly the car started, but was giving the same glow plug errors except now in cylinder #4.
• The car continued to run and start fine for a couple weeks, despite the OBD errors.
• Then one day after those couple weeks, just cranking but not starting again.
• This time I tried ordering a cheap glow plug (https://www.ebay.com/itm/112318375313), and replacing the #4 plug with the new one, but still wouldn't start. Tried rearranging plugs, and still no start.

During all of this testing, even prior to replacing the bad plug, I do imagine that I wore down the battery with all this futile cranking, so I hooked it up to jumpers and tried starting it that way, but still just cranked with no start.
Is there anything else I should try before getting it towed to the dealer for diagnosis?
(I'll be doing the buyback program shortly, but I know I gotta get the thing running first!) Thanks!