Thanks all for the input.
CDRU is bang on with where and how it is freezing... prior to my previous sump pump failure, I only had a single pump without a check valve. The stand pipe would drain back after each pump out and, effectively, double the number of pumps required to maintain the pit. That said, it also eliminated the freezing issue but increased the possiblity of an air lock on the pump.
I bought some Big-O end caps last night and stuffed the vents full of pink insulation before capping with the end caps. I do not want a permanent solution plugging the hole as they ventilate humidity in the summer. For those who aren't familiar with this type of setup, this 'room' is the area under the front concrete porch. It is separated for the basement proper by a steel insulated door and the room has no source of heating or cooling. Basically, it normally stays about 4C in the winter and about 12C in the summer. This has been an exceptionally cold winter but I want a solution that will be permanent.
When at the HoPo last night, I looked at the heater tape and spoke to one of the 'plumbers' at the HoPo (remember when Home Depot used to have knowledgable workers... it seems sometimes now it is whoever can fill out an application although some are still very good). The cables indicate they should not be used on drain pipe (supply only). I expect this is due to a fire hazard if the drain is empty and the heater is running? Comments? Someone with more knowledge than the 'plumber' at the HoPo?
Hoping a permanent solution is discovered before this cold snap stops and I have no way of testing whether anything is working