Backplanes are handled transparently by the SAS controllers. Be sure to use SAS 2.0 HBAs and Expanders for the most flexible, easiest configuration. FreeBSD is only vaguely aware of what's going on, and it certainly doesn't "need" to know.
Multipath, use geom_multipath and it's configuration utility
man gmultipath for more details. Ask specific questions if you have any problems, but the "automatic" configuration is pretty darn straight forward when using SAS disks (and even modern SATA disks with SAS controllers).
The light on the front is going to be much harder (along with all the other management functions)... Different "enclosures" have different management functionality and methods of accessing that functionality. There's SES (SCSI Enclosure Services), SESoI2C, SPI, SGPIO, SEBM (for SATA), and possibly other proprietary/custom solutions. I'd have to really dig into your setup to tell you any more than this generic stuff.
Slight side note: the utilities
setobjstat are user unfriendly tools that can usually read/write to these enclosure services. They depend on the enclosure or controller supporting the standard mechanisms though. Neither are part of the standard world as they're a bit half baked and need cleanup. iXSystems has worked with the community to develop these, they're good enough to be backend utilities to a web-gui, so they've stagnated a bit.
Also, that card you picked out has a Marvell 9480 chipset. Marvel is not open source friendly, quite the opposite really. Stick with LSI or Adaptec, both companies have actually contributed code to FreeBSD. Most cards are extensively functional at the minimum, though you should consider FreeBSD's HCL when picking cards/chips.