If Bee already has a good relationship with the audiologist at Costco, it would seem logical to follow his advice. The only risk at Costco is the tIme required to purchase and receive the hearing aid. In the grand scheme of things, the TV streamer, although not cheap is well worth purchasing from Costco. If after making the purchase at Costco, Bee decides her purchase is not providing the expected results, get a refund and try something different. She just needs to pay attention to the return window. If she likes the aids but not the streamer, she can return just the streamer.
Also remember that Costco can fir Westone ear molds for a fraction of what an independent audiologist will charge. With the gain that she will require, I would expect the ear molds to make a huge difference. If they need to be remade, Costco will remake for no charge.
I understand the desire to save money on the older iPhone, but remember that Apple is cutting off support for older phones. I would suggest looking at a newer model.
BTW - The Oticon App on iPhone pretty much negates the need for a remote and is far easier for elderly to master. The Connectclip is normally only useful to iPhone users if there is need to make a connection to a Bluetooth device such as a computer. Again, if she is interested, she can purchase and try it with the right to return as per Costco’s policy.
Keep us informed as to her chosen path and good luck.