Hello,
SWAT+ always (for the upcoming years, anyway) will be enhanced. Define "development", as there are always new releases and updates.
Yes, it is possible to use SWAT+ and any tool to set-up and calibrate your model. Define "officially"? :) You can find the latest version of the tools on the official site:
https://swat.tamu.edu/software/plus/No one can tell you what is better, it all depends on your are of interest, case study, data, timing, etc.
I can see that most projects are using SWAT+. Probably most older projects use swat2012 and are now being published.
One disadvantage of SWAT+ is the limited documentation, which will be remedied quite soon.
Hope this information helps!
Best,
Natalja