Just checked out your post in C&V forum. So a few more idesa.

Lights (and leds more than bulbs) will flicker when powered by a dynamo at low speeds (hub dynamo will cause more flickering than bottle dynamo). One of the solutions is to ride faster. But there may be a particular issue with your setup. You have 25 leds in parallel. I don't know how much milliamps they need each, but there is a good change that 25 of them are not getting enough power from the dynamo hub. This would be solved if you put 12 pairs of series leds in parallel as I mentioned in my previous reply.

Adding a capacitor or batteries will equally help. But when you use batteries make sure that you use some resistors if necessary to limit the current through your leds.

But your AA batteries will not explode under any normal circumstances.