Yeah fill in that old vent asap. It likely is the source of the issue.
Dunno if a 100-watt bulb is worth it. I would go with the pipe cable. You only need a very short length of it. See:
»www.homedepot.ca/product/3-ft-el ··· e/909324
»www.homedepot.ca/product/6-ft-el ··· e/909325
Pennies to run.
Or just remove the check valve and adjust your floats on the pump for a higher water level. your pit look like it can accommodate that with no issue there. It will just raise the water level by maybe 4 inches or so.
Getting back to diameter. It sure does look like they are the same size. Going into and out the wall should be a minimum of 3 inches but I've been reading people use 4 to help prevent freezing as you are having.
For this winter, stuff that old vent with pink insulation or similar and spend the 25$ on the pipe heating cables or remove the check-valves. to solve the issue for the immediate.
I'm thinking in the long run, that if you remove the pipe from against the wall and run it straight up from the pit (against the shelving) and 3 inches higher than the exit, then run a pipe at an angle to the wall exit that this should add enough buffer to prevent freezing and keep it off the wall.
Yeah sure it won't be as pretty, but it will likely prevent this issue from occurring again since you indicate it occur at the exit at wall, only.
So my final answer would be that. If you don't want ugly, then stick with the pipe cable for ~25$ (get 6 feet minimum). Should be a lot less cost to run this than a 100W light bulb. Think this cable is only like 12-watts. Might cost you the same price as a coffee to operate for the season.