Hello:

Castlegar is often a challenge.

Check the weather at Kelowna and to be safe, Vancouver. Kelowna is a shorter trip; however due to inversion can be fogged in on occasion.

If Kelowna looks good, that would be your best option.

If you can't fly due to weather, AC should work with you. But not clear from your post what your expectations from them are. I.e. if you have a ticket to NY from Castlegar via Vancouver then they would likely work with you if you did a portion by bus. If you have a ticket to Vancouver and a separate trip to NY the options might be different.

Kelowna also has flights direct to the US on Horizon. Good luck.