There is zero chance your lighter will damage the wood, unless you've got a clarinet made of bamboo :-) . Grenadilla wood is next to impossible to ignite. As practical matter, matches or lighters are often used to soften pad-glue and reseat pads in situ.
I would stay far away from hammers, as you're far more likely to bend things and make it all worse.
Now, WD40 is not really a lubricant. You might try silicone spray, or any of those products specifically designed to loosen stuck screws. These typically recommend applying a little and waiting 30-60 minutes to take effect. In all cases, take care to wipe off any excess fluids to avoid any possible damage to wood or to nearby pads.