If Mainline is out of stock, try USA Airbrush- they usually have a good inventory, and have been my go-to for Modelflex for 10 or 15 years. The caveat being that they sell paint and airbrushes, and no model trains to fill out an order, so if you only need one bottle of paint, the shipping will be more than the cost of the paint. Also, since it sounds like this color is mission-critical for you, best to check with them on stock before you order.
https://usaairbrushsupply.com/shop-our-store/ols/products/16-02-reefer-white-69ce94On the other question.... my best advice is to not mix between brands. If you have to, try mixing a small amount, to see if it will mix well or react in some undesirable way.
Has anyone yet come up with a Modelflex mixing chart for all the colors that Modelflex DOESN'T make? I've played mix and match myself, and some standardized info would be very helpful.