Hey I'm installing a moen cartridge for the first time and having trouble with getting the retainer clip back in

I lightly toothbrushed the faucet with a mix of vinegar and water and let the water run thru to make sure the cartridge would go in all the way

But it seems like I don't have space to fit in the clip

It's supposed to go thru the slit part of the valve, right?

This is the same clip that was in with the old cartridge (also moen 1222), I unfortunately didn't take pics while removing parts

Should I cut the tile above? I feel like that's a bit extreme, but I think a plumber we had before knocked out a whole block in another shower, maybe because it was like this

Thanks for any advice and Happy Holidays !

Hoping a new post brings fresh ideas. I have a 25 year old Moen positemp bathtub /shower faucet. It's been dripping for years so I finally decided to tackle it. I had waited because there's no shut off valve and I was afraid to shut off the whole house in case I screwed it up. Got my cartridge 1222 from Moen. Couldn't get the old one out so I got the pulling tool from Lowes. I got it out and it came out so hard I fell back, glad the glass door wasn't behind me.

Could not get that darn clip in. Stuffed a paper towel with vinegar in there for about 20 minutes and tried to scrub it with a little brush for straws that I have (not a wire brush) and I did turn the water back on to blast anything out for a few seconds.. Still could not get it in. I even used the tool to shove it in as far as possible. I put the old one in and it almost wouldn't go in, had to use the tool to get it in. Finally, after a wiggle with the tool it snapped into place, figuring that was the trick I tried it with the new one but NO go. I put the old one back in and at least I can turn the house water back on.

I saw a video, probably by a plumber, where they had the perfect round copper brush that they stuck in there...is that my only hope? It doesn't look gunky, but I think there MAY be a little build up around the very perimeter of the pipe in the very back. I don't know what can clean, or if that is the problem. I need 1-2 mm more. Would CLR work better than vinegar? Is paper towel ok? Do I need that stiff wire brush and does HD carry it? I'm so close!! This should have been a 15 minute job, took all afternoon including the soak, trip to Lowes and internet research. Thank you for advice!!!