The center bolt on your spring pack is fine. Do no attempt to remove it or replace it. This will get you into a lot of work that is not necessary. When everything is back together the U-bolts will help hold the springs together. The top of the bolt is round to fit in the round hole on the axle. This ensures the axle and springs are in correct alignment.
If you do not replace the U-bolts, at least clean the threads. A wire brush works good for this. A spring shop can make new U-bolts and supply bolts for you if one is close by. Next if you do not have a torque wrench, borrow one. Most chain auto parts stores will loan or rent tools. Not sure you have the torque specs for the U-bolts on your trailer. If not the axle should have manufacturer info stamped on it. Then check the internet.
Getting the frame jacked up should take the pressure off the axle and allow it to rest on the spring pack. Jacking up the frame will change the weight on the other axle causing the spring pack you are working on to move.
Also do not remove the springs from the shackles. The other axle will push or pull it out of alignment and you will be in for more work trying to get the holes lined up to get the bolt back in. I speak from experience when I replaced the shackles on my lowboy trailer.
I would not leave it on jack stands for too long. I always worry that something could cause the trailer to shift and fall. Also do not get under the trailer when you are tugging and pulling on things.
Good luck.
