My suggestion would be to send an e-mail to Costco Canada via their “Ask a Question” system. Identify the store, the fitter, and the concerns you had about misinformation. I’m sure Costco would want to know about an employee giving out bad information, and making claims that their premier hearing aid product, the Kirkland Signature 9.0 is not available, and especially being told to buy a hearing aid elsewhere.
I believe the Phonak branded product they offer is the Brio 3. I wonder if you were given one of those that happened to be in a M70 box?
With respect to power, typically all brands come in 4 power levels from standard to ultra power. This includes the Rexton and KS9. When I look at your loss in the Rexton software the M receiver is a bit under powered, but should have lots of power with a P receiver. Here is what the gains look like with a Rexton P receiver, and NAL-NL2 prescription formula, and closed click sleeves.
Your right ear should be OK with respect to feedback, but the left ear may be a bit of an issue as you only have about a 5 dB margin between the soft sound gain and the blue shaded feedback potential area. If you decide to go ahead with an aid, I would try it first with a closed sleeve, and if it does not work out for you, then have them order a custom mold for the left ear. You can get your money back for the mold if it is not to your liking. They will remake it also based on how it works for you.
I would tell them ahead of time that you want to try an aid with the REM done, and if it is a Rexton with the own voice processing turned on. It really does not take that much time, and you are correct that if they do not do it, you will not know how the hearing aid is going to sound to you. Now if you want to try multiple aids, then the time will start to become an issue. You may need to do it in a couple of appointments.
Oh, and in general, lower level, and lower priced hearing aids do not cut back on power. They most often just have fewer bells and whistles.
Hope that helps some.