Codeshares are a deceptive marketing practice that have no upside for customers. Sometimes the downside may be limited, in which case it may be fine to book them, but I would never go out of my way to book one.

I think you mostly have the answer from others as to how to do this. The one thing to note is just to make sure you can ticket the AV fares on AC stock - some airlines have restrictions, especially on cheap fares, on who can ticket them. With AV, it's generally not a problem, but in my experience, it can be difficult to ticket AV flights with AC, so it might be worth spending a bit of time with EF to check.

The good news is that AV often has some quite good J fares and prices most things as one-way, allowing for interesting combos. For example, BOG-SCL; GIG-YYC (via BOG & LAX) recently cost me $4000. Intra-Colombia J round trips can usually be had, if booked a few weeks out, in the $200 range. And so on.