Hello Kelly,
I can answer some of your questions:
Your common plant might be desirable for someone else, so yes, donations are welcome. I am part of the seed team, and when we are processing the donations ready to send out to the volunteer packeters, I am often surprised to see that there is only one donor for lots of the taxa. So if that donor is unable to donate next time, there will be some disappointed people!
We accept a wide range of seeds - it could be difficult to decide what is or isn't an alpine! (we might draw a line at cabbage seeds, though!) If you look through previous lists you will see entries for beauties such as Acer griseum - I have a sapling of this in my own garden, thanks to the seedex. So yes, tree seeds are welcome too.
How many seeds as a minimum donation - I have seen donations of special things with just half a dozen seeds. Send in what you can - none of the seed goes to waste - for example, after the seedex, spare seeds are sent round the Shows for sale to the general public. This raised some extra cash for the club.
I don't think I have come across people sending in extra seeds from commercial packets, so I will leave it to someone else to answer that. Such seed would be at least a year older than your freshly-collected garden seeds, and in some cases, would result in lower germination rates.
I hope this is of some help and will encourage more forum members to join the SRGC and take part in the Seed Exchange. It is well worth doing - I have had some excellent seeds over the years, almost unobtainable elsewhere.