If I exchange 2k gold at once, and then later exchange 1k gold will I get the first 2 tiers again, or will it add up to 3k gold towards this event? (Sorry I have not exchanged in a while so i cant remember)
All exchanges during the event period count towards one single total amount. If this amount reaches the top tier, then any extra Gold will start over, counting towards more tiers.
So, in your example, the total Gold would be 3k, meaning you get Tiers 1, 2, and 3.
Hope this refreshes your memory.