Chelle, I have severe allergies and metals are one of them. They weren't sure if I was allergic to the titanium or not, and my body went through some major unexplainable swelling and then things just got better. I am thinking it was not the titanium at all, but the BMP that was used, which is notorious for causing swelling not just at the surgery site, but all over the body in some. The only way to tell if you are allergic to the titanium is not by a "skin patch test", but by blood testing, which is done at very few labs in the US. Deep implantation of metals verses those that contact the dermis of the skin are very different and a person can be allergic to a metal on the dermis, but not when deeply implanted into the body. Titanium is said to be the least reactive of any metals for deep implantation.
You are right, some people's bodies just dont adjust to "foriegn matter", which the hardware qualifies as. But it could be your screws are just not holding well enough and often does not show on imaging, until they go back in surgically and find the hardware literally is just sitting there and/or ready to fall out.
The hardware does need to stay in tact until you are 100% fused if possible. Then if you are still having issues and they can't determine what else could be causing it.. some doctors recommend removal of the hardware. And in some.. it's made all the difference for them pain wise. Others not.
Before you go the removal route, you should get a second opinion before opting for additonal surgery.
BTW, my muscles were not cut either, but dilated through instead, which made my healing of the tissue a breeze and very little to no scarring. I twas all for the dilation verses cutting the muscles.
Hope this helps some. Hang in there and give it more time. You could just be a very very slow healer. I am normally a fast healer but not with fusion surgery. It's taken me the WHOLE TWO YEARS to heal from this.