Barrelcactusaddict,
As Lou Jost points out, it matters quite a lot that your subjects lie deep within an amber block. I say "deep" because 5-15 mm is plenty of distance to wreak havoc with image quality if the amber is at all non-uniform or has a surface that is not perfectly smooth, planar, and perpendicular to the lens axis. Under ideal conditions (perfectly uniform amber, with a surface that is smooth, planar, and perpendicular to the lens axis), a 5X NA 0.14 objective can look through 10 mm of amber with no problem. But a 10X NA 0.28 objective will be hazy, and a 20X NA 0.42 objective will be
very hazy, almost unusable. Introduce any non-uniformity, or surface roughness or curvature, or a slightly off-angle view, and all the images get worse, much more so at larger magnification and NA.
So, your fundamental problem is the amber.
You have written several times of wanting/needing 10x-20x. I guess this number comes from wanting to image a 1-2 mm inclusion across the entire width of your camera's sensor. But in any case, thinking of "10X-20X" is the wrong target. Instead, you should be thinking about how you can get the best image possible of a 1-2 mm inclusion, behind 5-15 mm of amber, for whatever your budget will allow.
At this point in time, my read is that the Laowa 2.5-5X ultra macro is the best lens for that job. It is significantly cheaper than the Mitutoyo 5X, even from Lightglass Optics, and it offers the huge advantage of providing an easily adjusted aperture, which can be used to optimize image quality in the presence of imperfect amber. If you want more magnification, to put more pixels on the same optical image, then you might add a cheap teleconverter behind the Laowa. But don't do that until you see what you can get from the Laowa by itself.
I'm just so disappointed in myself, and sick to my stomach thinking how much I spent on equipment, all under a false knowledge of how lenses work. I don't know what I'm going to do with it all.
Again, the first thing to do is to take several deep breaths. From what I see so far, you probably got the 200 mm lens used, so you can resell that for essentially the same price. Nobody tracks number of owners on these things. The specialty thread adapters are worth almost as much used as new, since they don't wear out, so the major loss there is shipping. For starters, list them in Equipment Exchange here at PMN.
As for false knowledge of how lenses work, yeah, that's a problem. Maybe we can fix that with some further discussions. But when you ask "would using just a 5x infinity-corrected objective give the same magnification when used with any lens set to infinity", after I've written the explanation at
viewtopic.php?p=277617#p277617 , I have to confess that I have no idea how to fix your understandings. Perhaps the explanation given by JKT, in the previous reply, will do the trick.
--Rik