You are correct, all the traffic that all of the access points gather is going to be tunneled to the wireless LAN controller, no matter the VLANs or the SSIDs that you have, the WLC is the device that is going to tag the packets and send it to the switch so you have to make sure that the switch is properly configured with all of the VLANs that you configured on the wireless LAN controller. The only setting that will override this behavior is the HREAP setting with local switching which will keep the traffic locally where the access point is not going back to the controller.
Also, the Link Aggregation feature will work the way you described it, it will load balance the traffic on all of the active ports on the wireless LAN controller so if you have all 8 Gigabit ports connected to 8 Gigabit ports on your switch you will have a big 8 Gbps link between your wireless LAN controller and your switch.
2504 is a different story, it doesn't support LAG so all the AP traffic dumped onto one physical interface only, only the uplinks can be distributed. it is a low cost software based controller. however, it will perform well at its supported AP and client limit also haven't heard anything abnormal about it if used for its purpose.