Canada doesn't seem to say anything much about US passports. I can't even find something that says whether a visitor's passport -- of any nationality -- must be valid for a certain period of time when the visitor arrives in Canada.
If you look at the information for international visitors, there is a link to a page purporting to give entry requirements by country. This isn't particularly helpful for you, however, because for US citizens it simply says
U.S. citizens must carry proper identification such as a valid U.S. passport.
The words "carry proper identification" are a hyperlink back to the first page.
Now in the past, Canada used to be fairly explicit that US citizens could enter as long as they can prove both their identity and their US citizenship, and that this could be done using multiple documents, such as a driver's license and a birth certificate. This was even true for some years after the US began introducing stricter documentary requirements at the border.
Over time, Canada became increasingly vague, recommending that you use a passport but making it difficult to determine what the absolute minimum requirements are. The lack of clearly stated requirements concerning passports suggests that any passport will do.
An eTA application can take up to 72 hours, though they are usually granted within a few minutes. Some peace of mind is probably reason enough to spend CAD 7 on an eTA application with your Croatian passport, but it looks like you would probably be fine with your temporary passport. At this point, it's all about your risk tolerance.
If your temporary passport works, it will be a bit simpler to use it for travel in both directions, but it's not particularly complicated to travel to Canada with the Croatian passport and back with the US passport, and it's certainly not illegal. There is no penalty for violating the US law on the use of US passports by US citizens, and furthermore you would not be violating it: It does not require you to "use" a US passport when you leave the US, only to "bear" it.
