Usually my tomato
seeds germinate within 2 days days at the most with cool temps. When in the 60's they germinate very fast. In the 70's don't use heating pad.
With other seeds with harder outer shells i will usually slightly scruff the outer shell with a fine piece of sandpaper.
Again if no germination with the rockwool damp paper towel will do the trick.
As for brussel sprouts I have never tried growing them so I would try 1 plant the fist time and judge by that if you could do more the next time.
Yes those rails will be filled with roots after 1 grow it's incredible how much comes out. With rails its better to get the roots out as soon as possible otherwise the roots will stick to the sides and bottom. I use a power washer to clean then out it's so much faster.