As you are already finding there are many possibilities. All you can do is use a logical process of elimination. Be carefull regarding the compression/leakdown test results. You can have good cylinder compression with bad
oil rings. In some cases
oil in the cylinder can actually raise the compression. You are wise to monitor the
oil consumption as a baseline. Oil consumption doesn't always show up as residue at the tailpipe. The vacuum test is also very good advise. The oil on the spark plug threads is a big clue We all have a tendancy to think the worse at first. Start with simple than work you way down the diagnostic tree. You have a major puzzle on your hands that I believe will lead to removing the head. Let us know. You have all of us very curious..
Thanks, Matt