I think before you try to come up with ideas on how this would work mechanically you should look at the loadings and what size structural members as a simply supported bridge you would need to use to support a vehicle over the gap.
If you do that you will be able to see what you can do with the beams you need. What loadings you will be putting on the support wall during the cantilever operation of the bridge support and deck. Them you would need to see what improvements you might need to do to the wall to support the bridge in the cantilever position as it looks like a blockwork wall to me.
If you do that you will be able to see what you can do with the beams you need. What loadings you will be putting on the support wall during the cantilever operation of the bridge support and deck. Them you would need to see what improvements you might need to do to the wall to support the bridge in the cantilever position as it looks like a blockwork wall to me.