Upgrading OSX on a three partition hard without losing bootcamp
a) how to get bootcamp startup disk back? 

You have an additional 24.9 GB partition between the "Mackintosh HD" and the "DATA" partitions. This means you have 5 partitions (if you include the hidden EFI partition). The "BOOTCAMP" partition is the 5th and last partition. The Apple software is designed by default to search for Windows on only the first 4 partitions. This is why you can not startup to the "BOOTCAMP" partition. 

The easiest fix would be to delete this partition using the Disk Utility application. The space could then be added back to the "Mackintosh HD" partition. I have not used the OS X 10.6.8 Disk Utility application in a long time, but with the newer OS X versions, sometimes this application can fail to properly delete partitions. So, beware and you probably should have a backup of important files.

b) how to upgrade to el captain, maintaining three partitions and the windows installation?

I am not sure why El Captain would not install. Are you sure you first upgraded OS X to version 10.6.8? Also, I do not think shrinking the "Mackintosh HD" partition by 1.5 GB would help. If anything you would need to erase files to create more available storage. I believe you need 8.8 GB of available storage.

It is possible to maintain the three partitions, but requires an effort. The problem is Windows can only be aware of four partitions per HDD or SSD. For your internal drive, you currently only have four partitions. (The first partition is a hidden EFI partition.) Installing El Captain will introduce a fifth "Recovery HD" partition after the "Macintosh HD" partition.  There are three ways around this problem:


Delete the "Recovery HD" partition. You can optionally first copy
this partition to a flash drive if you wish. The problem here is
that some later update will probably recreate the partition, so you
will have to do this again later.
Use some free third party utility like gdisk to manual set which
partitions Windows can access. The problem is the Disk Utility may
reset the manual changes back to the defaults, requiring you to
manually set them again later.
Put the Windows and FAT32 partitions before the "Mackintosh HD"
partition. The problem here is that you need to reinstall Windows or
use a third party tool to move the partitions. If you reinstall, you
can not use the Boot Camp Assistant to create the Windows partition.
Personally, I chose this method. You can use the Snow Leopard
installation DVD to create the partitions.


Note: If during installation you are asked about Core Storage, I would advise to avoid using it. You do not need Core Storage unless you want to encrypt your disk.