Hi and welcome back 🙂
I answer only the part about keeping event items:
For certain events like the xmas and halloween of the current year, you can be pretty sure that they will be re-released in the following year. So you could keep those event items.
For re-released events, nobody knows when or if at all the end up in the shop. Again, the „fix“ events like xmas, halloween, and probably valentine, will make it there.
Players events usually do not end up in the shop any more. Ice events have been said to be one time only.
So there is not much of a definite promise with most events.
On some, you can rely on.