Ok, I went out at refreshed myself, thankfully I havent had to work on my car in a while, so I was getting a little fuzzy.
There are 3 hoses that connect to the coolant overflow bottle.
There is the small rubber hose maybe 4 inches long that comes out of a metal hard line from the engine, this hose connects toward the top/back of the reservoir.
Then there is the biggest of the 3 hoses, that runs about dead center out of the reservoir and connects to the thermostat housing.
Also the overflow hose which runs out of the top, takes a sharp 90 degree turn and then points straight down and connects to, as you mentioned sandman, it connects to nothing. If you have too much coolant, it lets the excess escape the system by pouring out on the ground. Kind of strange, but many vehicles have a very similar setup.
Now then, to get to that sensor, you will need to remove the 2 small nuts at the top of the reservoir, and then get some pliers and remove the clamp on the small hose that comes off the metal line. The first one I mentioned, just slide the clamp back far enough to where you can get the hose removed. It will leak a little bit no matter what in my experience but if you can keep the exposed tip pointed upwards it will help a little. Also if you do the repair on a level surface it will help. A third little trick you might try is finding something non-harmful that will fit tightly in that hose....I think you average round Bic pen fits in there just fine.
Now then, this is where opinions might start to differ, if it was me, Id pull up and out on the reservoir while leaving the main hose connected. Yes it will kink it slightly while youre working on it, but it should return back to normal with no issues, especially since you wont be in there long.
As for the wiring to the coolant level sensor, you should be able to disconnect the wiring harness from the sensor. Dont remove the sensor from the reservoir, or as you stated, you will have a mess.
With that small hose and electrical connection disconnected you can basically either set the reservoir on the battery/fusebox area, or on top of your engine cover, whichever seems easier to you. You CAN absolutely disconnect the hose to the thermostat if you want, but to me its just more mess and more trouble, it wont hurt that hose to be bent for 5-10 minutes. If it DOES hurt that hose, it needed to be replaced anyway
Other than that it sounds like you know where the sensor is, it will be staring you in the fact once you remove that coolant bottle. Its held in by one screw, shouldnt be tight at all. Just disconnect the wiring harness. Then remove the old one by whatever means necessary. Probably grab, twist, and pull. Don't "hulk out" on it too much though, because the plastic parts could very well be brittle, and break off, making a simple project a big pain.
Sorry for the long-winded post, I was trying to be thorough and paint you a word picture haha.