As for a penetrating solvent / loosener I much prefer PB Blaster to WD 40. It works like Voodoo.
If clearance will not allow the tools from above...... slot the bolt with a dremel and get a B--F---ing screw driver.
Naturally you will want a new bolt regardless of what you use.
I have made a habit of applying a dab of assembly lube on things to fight corrosion and other issues with frozen bolts, screws, and nuts. I have having frozen nuts!
