I had the same problem with the 1.5L in my 95 Civic; the problem was caused by too many years with too little attention to the valves. As a result, a thin film of sludge built up in and around the screw/nut of each valve adjuster. Like you said, the screws and nuts seemed welded together! If I held the nut steady with a wrench and tried turning the screw then the slot in the screw would deform. The solution: heat each assembly with a small propane torch for 5-10 seconds, being careful to keep the flame moving over the entire screw/nut assembly. This will cause the nut to expand slightly, and will also soften the sludge. You will then be able to turn the screw while holding the nut steady with a wrench. It wouldn't hurt to take the screws and nuts off at this time and clean them thoroughly before finishing the job.