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Good questions. Here's what I would do:

1. This one, I think, is a shortcoming of E+. Air walls are a new addition, and I think there is a good argument for at least allowing the SimpleOpening object to be used. If you put in an issue on the E+ repo it can be looked into. I'd argue that this is potentially a bug, and those can often be dealt with quicker. The next best thing (for now at least) would be to use one of the power law objects. Those are usually used for smaller openings, but can also be used for large openings if you are careful and two way flow is not called for. With larger temperature gradients, one-way flow is not very realistic.

2. It's possible to convert between the various power law forms as you are doing, but be careful with the reference conditions. The crack form does includes corrections to account for non-reference conditions (at reference conditions the values are 1), so keep that in mind if you are doing anything with the reference conditions. I'd think it would be easier to just use the ELA object (as you mention), but this sort of approach will also be OK.

Basically, what you're doing is probably OK in a lot of situations. The hard part is going to be telling when it is not OK. There is some literature on this, if that's of interest I can see if I can find some of that. I'd definitely run some simulations and see if the airflow results are reasonable.