It's almost impossible to get the replacement cartridge back in. I finagled a C Clamp to get it in... and now 6 months later it is dripping.

There's a tool to extract a Moen cartridge, (see image) that I bet you'll be wanting in near future if you plan on doing your own work.
If you had used the Silicon Jel that came with the cartridge and lubricated the "O" rings it should have slipped right in. You do know that Moen warrantees it's products for life. Call: 1-800-BUY-MOEN (1-800-289-6636)
8:00 am to 8:00 pm Monday through Friday
8:00 am to 6:30 pm on Saturdays EST
Closed on Sundays and all nationally observed holidays.
To contact Moen Canada call: 905-829-3400
Ask for a new cartridge. If you have a problem click on back for assistance.
Good luck, Tom

Install the cartridge with the stem in the "open" position to let out the air trapped behind the cartridge. Otherwise you have to compress the trapped air to get the cartridge to seat.