If you added greater than 10lbs grain to 153f water, you prolly got down into the 140's temp range of the mash. Not sure if conversion happens that low.
Also, when you stirred your mash every 5 minutes, that will cause you to lose a LOT of heat. People stir the crap out of their mash if they overshoot their temps and need to cool it down quicker.
Heck, I'm having trouble keeping my mash temps up, and I insulate the kettle with an old puffy coat, and just let it sit for an hour.
Oh that reminds me, was your kettle insulated at all? And/or did you light the burner at any point to keep mash temps up?
Biggest thing to me sounds like temps started too low, and got even lower, causing little to no conversion. That and when you topped off the fermenter with regular water, if you didn't stir it at the time, your hydrometer was sitting in the "mainly water" part atop the actual beer/wort.
Dunno if I got anything correct with those guesses, but just throwing them out there in case they cause an "aha!" moment in hopes of helping you determine what went wrong. Good luck