I'm sorry I didn't see these questions....
I think there is such thing as an irrelevant leak, but I'm not positive on that. I've had idle issues that were carb related, not leak related. Usually leaks are found after the fact...after something catastrophic has happened, or incidentally during a teardown for other reasons. Most of the time, if a saw is running irregularly, i'll suspect a carb issue way before air leak. I'm a big fan of simply tearing things down sooner rather than later... a great opportunity to refresh everything.
can mods....etc.... thats quite a question. as soon as a saw starts running irregularly, it needs to be investigated or risk severe damage. rule out the simple stuff first, fuel, spark, compression. I usually start w/ a comp test. rule out fuel line issues, carb issues, air filter, spark screen, can be many things why saws don't run right. loose spark plug will do it too. hit the high points, do the basics, and if it still isn't running right, tear her down.
does that help?