By land or by air?

If by air, yeah, you need a valid passport (or Nexus card) to fly between US and Canada. The airlines simply won't let you board without it.

If by land, you can use an expired passport (or perhaps other docs you have, like birth certificate or other proof of citizenship and residency) to get into the US and back into Canada.

You'll probably get some questions and grumpiness about it (especially from the US side), and maybe even a delay if you have insufficient docs, but the US can't legally turn away its own citizens, and on the Canada side CBSA officers officially have a fair amount of flexibility as to what documents they will accept.

I personally have travelled out of and back to Canada several times by land, armed with my expired passport, Canada work permit, and BC driver's license. I've simply explained apologetically that my passport is being renewed and it's taken forever; haven't had any issues.

Given what I know now, and the fact that it took ~10 weeks from when I mailed my passport until they mailed it back, I would definitely renew at the consulate. (I also live a 10 minute walk from it.)

Yes. For an extra US$60.

But as others above have suggested, you stand a good chance of getting it back even faster than expedited-by-mail if you go there in person. Also, if you go in person you may be able to convince them to let you keep your current passport (whether already expired or not) while they're processing the renewal, so that you can use it to travel by land.