We have been many, many times to SB and we have never encountered a line at the SB BBQ. We were there 5 times this summer alone and thankfully never a line. I'm not saying it doesn't happen but it doesn't appear to me to be a troublesome problem.
Also, if heat bothers you, I don't suggest walking from the 2nd tram stop to SB. I have seen many people, me included, that got over heated. That runway is long and hot.
Chairs are plentiful even with the larger ships holding more guests. Umbrellas are a bit more hard to find if you get there late. Sometimes hammocks are very scarce.
In July, they were handing out towels two at a time. We definitely made use of them so don't worry about asking for two.