- if yes - shoud I do it? Any significant difference between WV and SWV/DWN? Is it worth to pay more?
I would upgrade to a SWV if you value a sunset / unobstructed view, more privacy and slightly less noise from the neighbours. Check out my WV vs SWV comparison. The link to the post is in my sig. There are also quite a lot of posts talking about the DWVs but you'll have to search for them. IMO it's not worth it to pay the extra for a DWV, not unless you've got plenty of cash to spare.
- do they still allow Diamonds to use their lounge in the airport on arrival/departure?
It's no longer free for Diamonds. Everyone has to pay the $55++ for the package. (See my post a few pages back regarding this for more details)