I'll confirm a few things for you as you've requested.

1. Forbidden techniques are indeed sealed in forbidden technique scrolls which are scattered around the world, where exactly they are scattered and how difficult it will obtain them has yet to be determined. I can only guarantee that it will be rare like every other special jutsu in SLO (chakra mixes, kkg) and therefore not meant for the general public but for the few that get their hands on it.

2. Forbidden techniques can and most likely will be sealed in more than 1 scroll, but don't expect it to be more than 10 either. Only a handful of forbidden technique scrolls will exist for a certain forbidden technique and these scrolls will have an immense value. They cannot be copied and you can only learn from the original scroll, that is how a forbidden technique is sealed away.

3. A forbidden technique master, or someone who has taught himself a forbidden technique through a forbidden technique scroll and still possess said forbidden technique scroll can take a teaching roll and pass down or teach his forbidden technique to other shinobi, without them having to own the forbidden technique scroll.