Hi @cupton,
In general, I would say that when you're using an external tool type of assignment (MH Connect in your case), you should probably leave the attempts in Canvas at unlimited and restrict the attempts within the external tool itself. This could be slightly confusing to students if they see the unlimited", but you could put that info right in the assignment description if you wanted to. The reason I suggest leaving it as unlimited on the Canvas side is in case the behind-the-scenes integration between the two systems ever has issues. The MH Connect tool may need to transfer grades more times than the allotted amount to get everything to be correct in Canvas, and the attempts could start to interfere with that. I'm not 100-% sure if MH Connect would be affected, but we definitely ran into tools that had an issue with this before.
Blackboard used to use a different integration type where info could flow back and forth perhaps better but required each vendor to do custom work. The Learning Tools Interoperability (LTI) protocol, which Canvas and Blackboard both use now, is more of a global standard, but perhaps not as tight of an integration in a sense that not every setting can go back and forth.
I hope this answer helps a bit, even if it's not what you would want to see in an ideal world.
-Chris