GPU SEEMS NORMAL( i just put the pci e x1 in the wrong side)
It can destroy your motherboard, but try to connect new one and check is everything ok?
Is everything ok with this bad one. But how can destroy the mobo
you can burn out a port on mobo if unlicky the whole pcie chip or you can burn some parts of the GPU as well happen to me once on gigabyte R9 280x happy it has been only a resistor and a small fuse before the data multiplexor chip, replaced them the gpu worked so i mined on it few weeks before i sold it on aftermarket but in case the pin is unused you should be safe
you just need to bring some 12V power on the data pins and bye bye 
btw try to clean it and replace it asap also you can track where is the pin connected that would tell you more
i think i burn one pci e when i put wrong side but i doesn´t test yet
wouldn’t supersize me before you test the port take a flash light( mobile phone for example) and check if you will see burned pins inside the port on the location if yes try to clean them contact spray but be gentle dont put there to much wait few seconds then take an ear stick and instead of your ears suck up the rest of the contact spray and again be gantle we dont want to bend the pins…you can also check the place around the connector if you dont see burned barks in case you have colored mobo a colour change may be visible as well
checked the photos you as suspected you shortcuted the 3.3+ power pin with the ground thru the rizer the burned pin on the rizer is ground irrelevant for mining funkcion but in case of malfunction of some componend (mobo, gpu.PSU,) it could cause more problems like you gpu burns out it will create short cut is quite possible that with this rizer it will take another slot with it since there is a delay until the PSU will turn off it self and a rezedial charge needs to go somewhere to ground and if it will go thru the pcie ground pins and there is one less the rest of the ground pins may not hold and you could end with another slot like this up to burned power controler on mobo or some part of it …as for the slot is melted plastic most probably residuals of the plastic are covering the metal pinn partialy i dont see it correctly from the photo i would check with a needle if the pin is holding its position (no wobbling to sides ) and put away the residual plastic if the pin isnt holding its place or has moved to side during the melt down forget it the slot is done even when the +3.3 has another pin as well when you put a gpu which will suck to much of the +3.3 line it will end up like this as well …in general i would forget on the port and in case you have not enough PCIe slots you can still purchase from aliexpres the PCIe extender card a spliter from 1 port to x2 or x4 slots even a x8 is available but not shore if it will work x8 work but the x4 works good (3 Gpus connected to it so far )
BTW link for the 8 port expansion card on aliexpress …hell their site addresses are long
Works if i put 8 por expansion card and put another on the riser?
i doubt it will work if you put extension card - riser- extension card i never seen such thing so cant tell if you have the extra money you can test it but i doubt it will work