The motherboard will support certain memory speeds and this specific motherboard is only designed to use memory up to 1333MHz as shown on the following web site:
The slot missing on the motherboard is not intended to be used and some parts ( additional from the port) are missing that are required to make the slot functional. Furthermore it will be very difficult for you to solder the motherboard without damaging other parts on the motherboard.
And for your last question, the PCI Express* slot is intended for graphics cards, but it is not limited to them. Intel(R) does not validate the usage of controller card on desktop motherboards, but we know that some will work. I recommend you to contact the manufacturer of the card and check if they have a tested motherboard list.
Thanks a lot for your answer!