Don't get too wrapped up in it, don't spend too much time, emotion, and energy on it. She's a big girl capable of making her own choices, and must be willing to accept the consequences of her choices.
My family has stayed and the Polynesian twice, and absolutely loved it. We have no basis of comparison, but would stay there again in a heartbeat even if it meant spending more than other resorts.
Here's how I'd present it.
First, it's a wonderful resort that costs more because it offers more than the other places.
Second, it offers two of the best restaurants on the property, but she doesn't have to stay there to eat at them.
Third, it has among the best transportation options anywhere on the Disney property.
Great views of the Magic Kingdom.
It offers two great pools, but I'm sure the Holiday Inn has a pool.
Finally, the Polynesion offers.... her family. If she wants to stay somewhere else, let her. Tell her it's OK with you, and if she chooses to stay someplace else, you'll try to get her grandchilden to call her on the phone every night at bedtime. Unless something else is going on and they're busy.
You don't mention how old your mother is or what her circumstances are, but I will offer you a rhetorical question my father once asked his mother when he caught her climbing in and out of a Barcalounger over the arm on the side without putting the feet down (she was convinced its predecessor only lasted 18 years because the footrest had been raised and lowered too much). You're welcome to remove his, um, eloquent "stylings" and replace them with your own declaratives.
He said "@#$% it Mom, you're 78 years old.... what the @#$% are you saving FOR?!?!"
There's are times and places for scrimping and saving. It's none of my business, but as far as I'm concerned a few days with the grandchilden at the Polynesian is worth the splurge.
Or you could really be nasty and tell her that it's ok, your children won't be lonely, because their OTHER grandparents will be there. Now THAT'S nasty....
Good luck, and remember, it's her choice. Don't let her take the wind out of your sails over this.
Have a GREAT time.