Most birds nails won't need clipping if you provide good perches... natural wood and java wood perches have varied widths and shapes will help grind the nails into an adequate length. I've never trimmed my birds nails and they've never overgrown, the only thing I do though is take the very very tiny tip off, literally about the size of a pin head just to prevent them getting caught on jumpers and such- its called blunting. They can still use them to climb and everything but it just keeps them a bit safer.
Most budgies and cockatiels shouldn't need their toes clipping (larger parrots are different!) but there are several bird diseases that can cause this and also elderly birds tend to have this problem. The foot should be able to rest flat on a surface, if it can do that the nails are fine. You will need to get an avian vet to show you how to do this.

Don't do it yourself as birds can't coagulate and they can blled really badly if the nail is cut too short.
If you give them the choice the will go to the seeds. Try well cooked brown rice, with steamed broccoli and a sprinkle of fresh chilli powder, let it cool to room temperature then remove the seeds and give them that. See how they get on.
Sometimes its about the right texture and temperature, you need to try everything grated, mashed, steamed, cooked, raw... but you can't keep the seeds in there at the same time. This will be a lot easier once they tame because ou can get them to eat from your hands. its often more about the texture with birds than they don't like veggies. How about fruits? Have you tried some mango?
Yes some birds will snatch nests.