Just to update the thread a bit, we've been PM'ing to get some more info. I know the tap water has 0 ammo, 0 no2 and +10 no3, and that the ammo is climbing up to .5 in a very short period of time.
First thing I'd do is use that new gravel vac and do a good cleaning of the substrate. There's a bit of bacteria in there, but I'd much rather get out leftover food and any bits of that pot that flaked apart in case that could be a cause.
After you added the SafeStart...did you stock the tank straight away, or did you continue to cycle with the pure ammo?
The two variables to me are the broken flower pot that was flaking away, and the SafeStart. If it was another brand, I'd instantly blame the cycling product...but I've never personally heard of bio-filter crashes with that product (only inconsistent results of it actually helping or not.)
My main priority now would be to keep on top of the water changes (luckily it's only a 10 gallon), get yourself some Seachem Prime (if you can find it) to use as your water conditioner, and go on a city wide search for seeding material. The only 100% proven thing for instantly curing ammo spikes is a nice, big, dirty piece of filter media from an established tank.
I'd let someone else chip in on this...but since you've already added SafeStart...who knows, maybe dump another bottle in? Bottled bacteria is not one of my specialties...but I personally wouldn't see the harm in adding in more at this point since it's already been used in the tank previously.