This is all possibly related to your scoliosis kindofabigdeal.
how is your posture are your shoulders, and hips unlevel.
Muscular sprains, and strains of the spine are a common diagnosis, but they have more to do with giving the patient an explanation for their pain that requires no further attention rather than finding out what the real problem is.
"I had a work injury 6 years ago which was diagnosed as a cervical sprain, and I was never given an MRI even though I had severe, radiating pain which did not respond to any of the treatments I was given"
This is pretty typical its possible you did some real damage only an MRI would show it but if an MRI did show some damage there may not have been much the doctors could do about it.
"Another doctor said I may have Marfan's syndrome because I'm tall and thin and my mother has back pain too"
Did the doctor follow through and test you for it if you have got marfans it would be good to know it however i know there is no definitive test for it.
Pregnancy would add more stress, and strain to your spine.
Yes you could have an inherited disorder but not necessarily so, conditions like marfans, and certain types of arthritis such as ankylosing spondylitis are often inherited. Has your mother been diagnosed with these or any other conditions that would cause symptoms like the ones you have.
As far as what the hell is wrong with you i cant say but scoliosis is almost always associated with some level of pain and the faulty mechanics that it causes, and the extra force on the joints can lead to accelerated degenerative changes to the spine if your scoliosis is advanced you may not have to look any further for a cause of your troubles.
MRI's are not usually performed during the first three months of a pregnancy and are avoided completely if possible but can be performed if a diagnosis cant wait.
i cant find any thing on spinal fusion and pregnancy but i would think that it is something to be avoided.
Good luck kindofabigdeal