Sorry to have got on your wrong side some-how dude.
I did not say you were wrong about your battery, I know exactly what battery you have.
I was pointing out a better battery is also a cheaper battery if you plan to buy another one. You might want to save a few dollars and get a better product for the job it's intended for. I know the deepcycle battery I mentioned with the black case is 115amphrs and costs less (here), works well. We don't now what the amphours are on the yellow case battery though and it costs more, at least at the Walmarts I been too.
I do know my Yellow battery was not as good as my black battery, which is why I don't buy the yellow for this type of use myself. I can't test the Yellow because it's wore out now, several years old, one of the oldest I had that was bought new. Mostly I used it as a starting battery and such, not wind or solar.
You are the one that is not sure on a few things, and I just offer help from experence. Since I don't know the exact numbers that would apply to you there I gave you the figures for me here, so you fill in your own numbers and figure it the same way to see what you need for your area.
You said, " I'm torn between buying another identical "marine" battery ($70), or just saving my money and buying some "real" deep-cycles."
The black case one IS a real deep-cycle and it costs $55 here plus tax.
Also I mentioned to be nice, if you don't have a dead battery to take back they charge the core charge, here that is $7, buy any junk battery at a scrap yard for $1-$2 to take in to save the core charge. May not be worth your time if only buying one battery, if buying several batteries it might save you $10 or more.
You said "The battery is a WalMart "Everstart deep cycle marine" rated at 875 marine cranking amps/675 cold cranking amps, no amp hour rating. I'm guessing it's around 80ah..? maybe less, I don't know. "
You are probably right or close, so for less money you can get maybe 35amps more? And the black case deepcycle should last longer also. I figured it up for you that the black case battery should run your 26watts for 5.3 days with no charging. Actaully it will probably run longer since battery amps are normally figured at a 20 hour rate. So if you run the battery dead over 20 hours you should get 115amps from it. The faster you drain it the less amps you will get and the slower you drain it the more amps you should get. So a 115 amp battery should provide 5.75amps for 20hrs steady. If you run 20amps an hour you will get much less than 115 amps total, and at 2amps per hour you should get allot more total amps. Heck that one 115amp battery might run your 26watts for 10days from a full charge to totaly dead in your use? About 8 hours on at 2amps, 16 hours resting time?
You also said, "I'm really not sure of the panel ratings. I'm guessing around 50w each." also "I need to get off my duff and get the 3rd solar panel added."
So you have 2 panels, maybe 50watts each, so you should have 100watts? I don't know what your average sun is for charging with 100 watts. But I think I figured it up for you that about 5hours solar insololation should cover 10hours a night with 52watts of panels? Then you need some extra wattage to cover battery loses, and after a day or more of little sun then you need to catch back up agian when the sun does shine, so that takes extra wattage above your nightly use. So my geuss based on 5hours is about 75watts should be good for 10hrs of nightlight. If you have about 100watts as you seem to think, then you should have plenty of panels (based on 5hours) so it would look like the weak point is your battery perhaps. If you only have 1hour insolation then the weak part may be not enough panels, you'll need to look up the solar insolation for your area to see how many hours you should have there.
If a good sunny day or 2 charges up your battery full after a few cloudy days, then you probably have enough solar, adding more won't help since once the battery is full no more can be stored and the panels are just shut off by the controller. Of course if your battery is never reaching full charge then more solar would help.
If your in the suburbs you probably have neighbors around that might be able to see your place, in that case a yard light at night is good because a neighbor might see someone stealing your lawnmower or such at 2am or crawling through a house window.
I simply stated that if in the country (like here and many of us) if no-one can see your house the yard light is not providing the security it would in a city. Since I know of about 50 such places just around here on back roads that is a valid observation that many people do not think about. I don't know where you live or really care. Lots of other people will also be reading this thread and some of them may realize their light is not as usefull as they thought, living out in a rural area 2 miles from the nearest house.
A trash heap is handy when you need odd parts and live 20 miles from the nearest store 
I just pulled the head bolts from a junk engine and turned them on the lathe to make other parts, beats driving a 40mile round trip and paying $5-$10 for raw stock if I could even find it. For most things it's a 100mile or 150 mile round trip to a bigger town to buy parts. So junk heap saves time and lots of money, Now when I have time to get the pistons out
Depends where you live, some places still have laws on the books that gives a land owner the right to shoot treaspassers. I geuss really I might be nice about it and the first shot would be a close miss for a warning, unless it looks like he has a weapon, then the first shot would be a good hit. I probably could not get by with that in Dallas or Chicago, but here I can and it's legal
Expressing an intent will not get you in trouble in general, posting a sign treaspassers will be shot is fine and in some areas even required.
Posting a sign John XXXX will be shot, now that would get you in trouble as premeditated perhaps.
Anyway it's great you have an interest in solar and nothing wrong with yard lights if you want them or need them, have at it. It's a great thing to have an interest in something, I by no means want to deter you.
As a mater of fact if you want all night lights in the yard, you could use a grid powered light with a light sensor near your Solar CFLs. At night when the Solar lights are on the grid powered will be off, when the solars are off the grid powered will turn on.
Myself I live in the country where I don't want them so I use my solar for more usefull things for me here, like lights in the house where I do need them myself and then I turn off the grid powered lights and save money on the grid bill.
