Thank you DEET.  I will try to source a lighting coil in the morning.  Just one question....when you say the destroyed lighting coil is likely the root cause of the problems I'm having, were you referring to the lighting issues or the engine missing/backfiring issue...or both?  I guess I don't know enough about how the lighting coil and source coil interact with each other to know if they are somehow connected in such a way as to make a destroyed lighting coil mess up how the engine runs.  Thanks in advance for your input; it is much appreciated.

Is your crank key in one piece> only asking as i expanded the pic right up to see there the source coil earth end goes, but guess over into the lighting coil as no visible end earth. Anyway just the crank key question. Also--performance--i see the the stator plate on full retard in the slots which i'd thing is unlikely so worry about that & timing & points gap combination after you've fixed the lighting coil. Also tighten the flywheel good as can damage the crank key if not tight. Key is only for locating & the taper 'should' do the holding.

Fix the obvious, then see what your symptoms are.

Thank you, RT325 and DEET.  I didn't notice anything wrong with the crank key, but I'll double check.  I did immediately notice that the stator plate was all the way to one side of the slots though.  Because I don't yet fully understand the electrical/charging/ignition system on the bike, I'm not sure I understand how or why it became positioned like that.  I do assume though that after centering it, the timing needs to be properly set.  I couldn't budge the three stator plate screws at all, so I hit them with some PB Blaster at 1:30 this morning (yes...I couldn't sleep just thinking about this whole fiasco) and am hoping they will come loose this afternoon. Regarding the lighting coil replacement...I'm really worried that the procedure is "above my pay grade", as they say.  While I'm more than OK with soldering (when there's room to do it!), I'm just so unfamiliar with this entire setup.  I found a lighting coil on eBay which I've linked to below.  While it says it is for a '76, it is the same part number as my '75.  The photos on eBay only show 2 wires (green and yellow) and I could swear there were more wires than that coming from the lighting coil on my bike when I looked at it last night...and I'm not even sure if there was a green and yellow. My wiring diagram shows there should be a "green/white", "green/red" and "yellow".  I'd certainly hate to screw this up, which is why I was looking for someone that did stator repair/rebuild work that truly knows their stuff.  In any event, I'll take a closer look today.  One question for now:  Knowing that I'll be waiting a while to get a lighting coil shipped, if I can get the whole timing & point gap procedure accomplished while I'm waiting, could the messed up lighting coil still affect how the bike runs?  This is the one thing I'm still unsure of.  Again...thank you all for your help. It's greatly appreciated!

UPDATE:  Looking at the lighting coil on my bike, I have a Green/Red wire attached (soldered) to one end and a Green wire attached (soldered) to the other end.  In addition, there is a yellow wire coming out of the coil as well as a wire that leads over to the source coil...and I just can't quite see where exactly they are connected to the coil.

That is not the coil for your bike.
 

DEET.....which coil is not the coil for my bike...the one that I described that is on my bike now or the one on eBay that I linked to above?

eBay

On post 45 it looks to me the solder joint is touching the flat spring on your points maybe grounding it out

OK...thanks.  Well, today was a somewhat productive day.  Several weeks back, I was made aware of a motorcycle salvage yard within "doable" driving distance from my home.  I went there today to search out a lighting coil and possibly an entire stator assembly with both coils intact.  To say I hit the jackpot is an understatement.  Unfortunately, I didn't have my flywheel puller along and only one of the many Yamaha DT engines there had the flywheel removed and the coil looked nothing like mine.The owner sent me home with another engine though that he thought might have what I needed. I looked up the engine number here:
www.cyclerestorer.com/pages/show_man_ser...&manufacturer=Yamaha
It came back as being from a 1977 DT250D.  I pulled the flywheel and took photos.  What you see in the attached photos are my current stator on the LEFT and the one from the '77 bike on the RIGHT.  My question....will this work with my engine (the entire stator or maybe just the lighting coil)?  The wire colors are NOT the same, nor is the plug at the end of the harness.  My harness has "Green/White", "Green/Red", "Yellow" and "Black" at the end of my harness, which is a 4-spade type plug.  The harness from the '77 stator has Yellow, Green/White, Black/White and a sky blue wire for the neutral switch all terminating in a different plug using pins instead of spades.  I know it's not identical, but was wondering if it might work with a simple change of harness plugs.  If not, I'll be making a trip back to the salvage yard with flywheel puller in hand to find a better (hopefully exact) match to my stator.  One more thing, and this may ultimately prove important...my engine's engine number plate has been ground away and I do not know with 100% certainty that I have a '75 DT250-B engine.  I completely forgot to mention this from the start, as I only remembered it this afternoon.  I didn't think it was overly important because my frame number corresponds to a '75, but I can see now that Yamaha made many small changes from year to year.  Thanks for any advice guys.