Electrical – type of extension cord I can use with a gutter heater

Without knowing the specifics of your wiring, you should be just fine running all of that and more from a single outlet. On one 15amp outlet I have running all at once a PC, PS3, 120watt AV receiver, and an LCD TV -- you'll be fine! If you do overload that circuit the breaker will trip or the fuse will blow, and you'll lose power to that outlet.

I would get a more competent electrician out to figure out what is going on with that outlet though.

It shouldn't make a huge difference but higher is better as you ideally don't want it to spend most of its time submerged. Using the provided clips is a better solution long run as this puts less stress on all the components of the system.