I would like to know if after Mod16 I still will be able to trade/mail things between accounts. I think I read somewhere that some things (enchanting stones?) would automatically become bound to account. Is that true?
No. Only items you covert at the Scholar are bound to your account. The items in your bank, inventory, and guild bank will not bind on pick up (BoP). The reason for binding is simple to understand. When you enter M16, you are allowed to go to the NPC Scholar up by the Rewards Claim Agent and trade enchantments for a box of any type of enchantment the same rank. Now imagine I buy 100 cheap armor enchantments at 20K AD in the current game, then wait. M16 drops and I go to the conversion NPC and get unbound Barkshield that sells at 250K AD and post mine up for 200K AD. Now do you think, that I would be the only greedy little monkey? Multiply it by thousands and it collapses the trade of armor and weapon enchantments. The unbound items you want to move from account to account, can be done by mail, it is only those traded at this vendor will become bound to your account.
It would be much easier in the future, for you just to buy character slots, and use the same account. My husband has a separate account from me, but I told him it would have been just as easy, to buy more slots for his characters on mine. I don't think you will have any issues in the next module.