Either side would be fine IMO, but it seems a convention to place it on the return side. Placing it on the flow side would make that side of the rad rather bulky with a thermostatic valve too.
You cannot use the electric element if water is circulating through the rad for obvious reasons. You do not however need to turn the rad valves off to use the electric element aslong as the pump is not running.
Obviously pointless to use it when the heating is on!
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