Thanks for posting your query at DoctorSpring.com
Firstly, I would like to comment that your query clearly shows that you are a very perceptive, intelligent and logical person.
The question has been so lucidly put and worded.
In all probability, what you have is sacro ilitis, and the acute bout was an acute flare up of it , which is quite common in pregnancy.
Also, proper diagnosis by imaging , joint aspirate etc is possible only after you delivery.
Yes, this is a disease with protean ( varied ) manifestations.
It goes into remission after childbirth and in some instances ( upto 20 - 30 % ) if you want statistics, it may flare up post delivery.
Having experienced what you have, I would not recommend going through the risk of VBAC.
Particularly since your doctors are not very vigilant about the sacro iliitis part and also because of their inexperience with low dose epidurals.
There is no point in risking major back injury and probable morbidity all your life, primarily since you are a caretaker and provider for your family.
Yes, an elective C section is a most legitimate, safe and sensible solution to possibly circumvent to the best extent ( I say this since even after childbirth you might have a flare up irrespective of the mode of delivery ) major back injury.
Childbirth and labour would most definitely put tremendous stress and pressure on the back and the sacro iliac joints, irrespective of the pain.
IN short , to answer your question -
There is a real risk of developing SI related problem during delivery ( more ) and post birth ( remote ). The risks of both can be more than halved by an elective C section.
Even if you are lucky and nothing happens during labour or childbirth, do not ignore the incident, and make sure you have extensive workup to diagnose what really happened that day.
All the best
Please feel free to discuss further.