Coming to this thread late, but hope this might help.
The bolt and shank of the bolt will be severely rusted in, as said before spray with Plus Gas or if you don't have any try dowsing it with paraffin and 3 in one oil - or mix up some engine oil and paraffin etc.(plus gas is essentially a paraffin and light oil mix).
As you have rounded off the bolt head, two options spring to mind:
1. Why not try putting nut back on bolt, tightenening up loosely and if there is enough thread left on the end of the bolt add another nut, undo the first nut to lock on the second, leaving a slight gap between first nut and pinch bracket. then use a ring spanner on the second nut and plenty of torque to turn bolt whilst whacking the top nut with hammer at the same time, this might dislodge the bolt enough to get it moving. Also hit the top head of the ball joint and underside loads to also ease the bolt.
2. Hacksaw a straight slot in bolt head, then use a chisle as a drift at 45 degrees on edge of slot to hammer the bolt head round. (needs two people though, one to hammer bolt head, and one to hammer end of bolt out! But as i re-call not much room to swing a hammer under the wheel arch so be careful.
When I did mine i had to buy a 2ft long extending socket bar to undo this nut, and give the bolt (with nut still on but on the end of thread only) plenty of welly with a hammer, to get it out, In my opinion the only way to get this bolt undone is to take the brake calliper off to start with as otherwise there is not enough room to get a socket on square - hence probably why you rounded the bolt head off.
If all has failled then put the wishbone bolts back in, nut back on pinch bolt and drive carefully for tomorrow. Go to VX to get a new pinch bolt and nut whatever.