I did something similar by using a heat gun (a really good hair dryer might work too). Just sit the bed on something insulate-y, then put the heat gun on high and heat up the vicinity of the solder pad for a couple of minutes, then solder it quickly after turning off the heat gun. Before doing this I had similar issues and had ugly solder pads as well... but get the aluminum hot first (hotter than is comfortable to touch), then the soldering iron doesn't have much more to go.
Regarding the OP questions:
1: don't need to worry about heating the plate enough to melt solder, it shouldn't ever get that hot (especially since the controller should limit it to 100-110C depending on how you have it set)
2: nope, 2 different sets of wires
3: maybe, but using approaches outlined above you won't need to
4: not necessary for PLA, but you should be able to and you may regret not doing it.
No need to feel embarrassed, just ask for help and you'll get all kinds of tips/tricks to get it going!