Filling or Root Canal? Care for a cavity?

[JessTheWorryWart]

I have a cavity on my lower first molar, however it isn't located in the center of the tooth, but more to the side--it's hard to describe without seeing it, but it kind of reminds me of how a piece of ice might chip off a glacier. It's created a kind of little scoop on my tooth. I had this cavity filled with silver (amalgam?) over ten years ago but it came out when I was eating a tootsie roll, so I had it filled with white stuff (composite?) Over the years the composite has seemed to be rubbing away, causing my tooth to indent where the cavity is. At this point most of the composite is gone and I am currently left with this scoop out of my tooth that is becoming discolored, especially lately. It seems to be getting deeper and darker. I experience a bit of sensitivity if I tongue it or chew on that side of my mouth, not really much sensitivity when eating hot or cold things. I am waiting to be approved for Medicaid so as soon as I'm approved I'll be able to make an appointment, but until then, I have two questions:

1. From the images I've seen of root canals, they seem to always go through the very top of the tooth. the Thing is, the top of my tooth isn't where the cavity is, it's more on the "ledge" of my tooth (picture the glacier again haha). Can root canals be performed in that area? And is there a way for me to know so I can prepare myself that I'm going to need one? It's concerning to me due to it's large area. I have no other symptoms, no jaw pain, head aches, sharp pains, swelling, honestly it's most concerning to me simply because it looks awful

2. How terrible is it that I won't be able to get this taken care of for at least two weeks? Could I buy that temporary filler so I don't have a big hole in my tooth? Or would that block off a possibly infected area and make it worse?

Thanks in advance