In most cases yes. Gold and shards can be farmed by doing Invader attacks as well as doing well on events (shards only).
No, but getting them to a useful point is painstaking and takes a good amount of time. The 4 types of dragon riders are always available and achievable, and I’m assuming you’re asking if they will be available when you get Atlas (?); yes unless something is changed or new content is released.
Defensive rider is a great bonus. If you select the skills to do so after leveling, the rider provides an additional 12% decrease in construction time on top of the current 25% reduction allowed by egg research. That added 12% seems insignificant until you see a Storage Hut upgrade drop by 4 days (approximately). It makes me feel less bad about not having the rest of the Green tier research completed to open the path to the final (Platinum egg) building time 5% reduction research.