I kept it semi-simple tent, cote (not for warmth but to be further from the water), sleeping mat (the foam one with reflective side) and -35C rated bag with liner, covering the ice is good too. I keep heater off when sleeping - more moisture, not very safe and not necessary with a good bag. One reason to keep it on is to keep hole and drinking water from freezing, but closing the hole usually works and for water - I remelt ice from cutting large soda bottles when they freeze (easier than melting snow and I don't trust boiling lake water here).
The coldest temp I actually tested my sleeping setup was below -40C in a backyard, and if you have poor sleeping mat water vapor from body makes sleeping bag to freeze to the mat and somewhat cool to sleep. So good mat is important. Ventilation is important too. Sleeping bag may have some frost on the outer shell, but it's normal, easy to shake off in the morning and doesn't mean it's cold to sleep.
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