Here are some more questions please:
1. Some rings, seems, are not touched by rust/corrosion . Why is that? Are they likely made from some special metal?
May be stainless split rings
2. Are freshwater trebles good for estuary fishing?
They will be OK but may be less well protected from corrosion than there salt water counterparts
3. Red trebles, seems ,are resisting better to rust. Is there some explanation?
They have a coating on them - possibly an alkyd resin coating or japanning process that offers some degree of protection.
4. Is there good place to buy small trebles in small bulk - 50?
Your local tackle shop should be able to help you
5. Do you wash Soft Plastic after fishing?
Not usually as they have been pushed on to a jig head and if left in place will expedite rusting of the hooks, so you take them off - but if you want to wash and keep them, then you will have to be aware that thet have allready had holes made in them. Short answer dont bother with this as they arent that expensive anyway.
6. I read somewhere in the past that is suggested to keep a piece of wool or small sponge with some oil in the lure box to reduce the humidity etc? Do you know anything about it?
Struggle to see how a piece of wool or sponge soaked in oil would reduce water vapour in the atmosphere in your tackle box - you could perhaps wipe your hooks / rings with it, but you would want to be sure that it does not create a scent that would repel fish.
Cheers Blood Knot