The only reason why Heavensward didn't have much problems is due to the brackets that were in place. Remember them? :P There was 3 layers of brackets. To help ease congestion. And didn't put huge amounts of stress on the servers. I remember this because I couldn't see the person that I was married to at the time, til I used my Eternity Ring to get to his bracket.
Square Enix can simply fix this problem by doing two things (maybe!). 1) Increase data capacity. Or 2) Enforce those brackets again til they fix every single problem that is incurring.
Both seems to be the best bet to do. There's a lot of data that is stressing servers needlessly. SE can fix everything within 1 patch or the day after patch by having another maintenance. 1-2 days is acceptable. But all these problems are needless.