Can one exchange SSD's in a single MBP for doing a back-up (delayed) on one of them? My logic board decided to leave without saying good-bye, MBP Mid 2012. This only serves as a delayed backup. I did however run Time Machine to backup my stuff, unfortunately it was 180 days ago... sure learnt my lesson now.

More specific, my MBP, when switching on, no sound, no display, only hear the fan running. Tried Target Disc Mode, no success. Going in for the replacement of the logic board soon and need to get the info off of the SSD as I read on several threads that if they replace the logic board with another make/version/model (which is the case cause Apple has a replacement program going on for the logic boards now), they will wipe the SSD and all the information on it will be gone. 

Can one SSD be swapped for another one in a MBP to backup data, and once the data has been backed up to a flash drive or external HDD, replace the first SSD and continue as normal afterwards?
Apple recommends backing up your HD as they "can't guarantee the integrity of the data on hard drive." Or words to that effect.

Theoretically you can do what you have proposed. You may have boot issues, EG the Mac the SSD is swapped into may not boot but that is unlikely to damage either the Mac or the drive, so if you have the Mac and the time, go for it.

Personally I'd put the SSD in an external case, plug it and your backup drive into the same Mac and make a bootable image file of your SSD onto the backup drive. It's a little easier and you are less likely to damage more than one Mac accidentally by digging around in it... IMHO...