Well, basically the idea is that when the dynamo isn't able to produce the required power, the battery steps up to take its place.

Years ago I had a dynamo light set-up on my bike with a small battery pack that had 3 (alkaline disposable) AA cells in it. When I slowed down, the pack switched to the batteries and so I continued to have lights when stopped. Soon as my speed picked up, it'd switch back to running off the dynamo.

Effectively it was a crude cache battery, without the charging ability. Once the AA cells were flat, it was dynamo-only. I'd imagine modern cache batteries do not have this limitation.

So in your situation, the tablet would continue to run off the cache battery even while you were stopped, at least until the battery was depleted.

As for capacity, I mentioned that the 2A would be a peak rating, and would include the power needed for the device itself and the embedded battery charging circuitry. Assuming worst case usage though, the 1000mAh would keep the tablet going half an hour, the 20Ah (20000mAh) would keep it going 10 hours.

The battery will either be charging or discharging, it can't do both. Basically if the dynamo is generating enough power to run your tablet and other devices, then the surplus will charge your battery. If there isn't the power from the dynamo, then the battery supplies the short-fall.