As I stayed in your other thread, I think it's a bad idea to stay in Coolangatta, but, if you've booked, then, that's okay, but, if you haven't, there is absolutely no reason why you would have to stay so close to the Airport. The bus that runs to the Airport even runs after midnight, so, if you need to catch an early bus, you should have no problems.
(1) The beaches won't really be "active" as such. You definitely can't compare Winter crowds to the numbers of people who go to the beach in Summer.
(2) With such a short trip, is there a particular reason why you want to go to Brisbane? I would personally prefer to spend all my time on the Gold Coast instead, but, do whatever you wish. There are always things happening in Brisbane - just depends on what you're interested in.
(3) Please note that the Gold Coast is made up of a lot of suburbs/areas, so, there's no way you can walk from Coolangatta to Surfers Paradise or anything like that. Make use of the public buses available. If you stay in more central areas like Surfers Paradise, that is where the main tourist things are like the SkyPoint Observation deck. Shops open a bit later there than anywhere else on the Gold Coast. You can also explore Broadbeach (area before SP), Main Beach (area after SP), etc.
There are beaches everywhere - in Coolangatta, Burleigh Heads, Broadbeach, Surfers Paradise, etc. Surfers Paradise has beachfront markets on Wednesday and Friday night, but, they are rather small in comparison to markets in KL, Singapore, HK, etc.!
Shopping malls include: Pacific Fair in Broadbeach, Robina Town Centre, etc. If you like factory outlets, then, you might look at Harbour Town. Surfers Paradise has lots of shops.
(4) Yes, you definitely should go to Dreamworld, especially, if you like rollercoasters. Dreamworld has some good deals - look at their tickets and passes to see which suits you best. I am not sure why you can't see the ticket prices for July, as they are all applicable for this year. There is the individual day tickets or you can look at passes like the 'Unlimited World Pass', for example, which if you plan to come back to the Gold Coast before June next year, then, that might be a good deal. See here: dreamworld.com.au/Visitor-Information/Great-…
Make sure to obtain the little piece of paper with the show times, when you get the Dreamworld map at entry, so, that you know when the shows are, and therefore, can plan your day around the shows you want to see. For example, there is a tiger show. Make sure to get there early because there are no seats. Instead, you will stand against the railing, so, get there early to secure a good spot, or else, you might have other people blocking your view.