03 seville sls won't turn over when jumping; car has been sitting for 6 months.

Probably a little of both. I am thinking that you are going to have to get a new battery. When you replace the battery be sure to get the key and put it in your pocket. The last place you want the key when you change the battery is in the ignition.

Agree,

So I brought a fully charged, 860 cold-cranking amp battery with me when I returned today. This time, when the ignition was turned to the ON position, more things started happening, like the radio turned ok n, ALL the lights in the dash turned on, the steering wheel moved. These are all things that did not happen before when jumping with the old battery. Despite all this, the engine STILL did not turn over, just the clicking solenoid again. No security light it anti theft indicator was in the dash at all. This doesn't sound like an anti theft issue, but could it be a seized engine? It's only been sitting for 6 months, and the previous owner just had the head gasket replaces right before parking it. According to the info panel on the dash, the oil life was at 74%, although in not sure how accurate or relevant that information is. What do you guys think? Thanks in advance.

The last thing left is the starter. It seems like you have done everything else.