+1 on both points.
This is dead on. At 5'10", 165# you are probably on the line between sizes. I am 20# heavier than you (in the winter when I'm fat) and I'd ski the Opus in 184 for sure. But at 165# you are reasonably lean and shouldn't be sinking the 178 (weight being the more relevant input in the sink v. float calculus - if height v. ski length were the issue you'd be saying stuff like "squirrelly at speed, not stable enough, not enough ski to engage. . . ."). But you are actually saying the opposite. "Heavy" and "awkward" translate to me as "last weekend this was too much ski for me to manage in those conditions, the way I ski." It just doesn't compute that a guy at 165# would sink the 178 Opus that drastically. Feel small? Maybe. Unskiable? No way.
Mount point could be an issue, but I wouldn't over think that one either. You should be able to adjust. That said, think that it totally depends on the ski and I disagree with the notion that a mount slightly negative of the line for a more directional skier kills the ski's purpose, "defeats the design" or suggests that you are on the wrong ski. That's hyperbole. I ski my Chetlers slightly negative of Team and they work great for me. All of the benefits of an uber-fun, surfy powder ski, but optimized for the direction that I am typically pointing. In my next life, when I am launching and landing big air switch, I'll move the mount forward. Until then, they work great. Go back through the threads on TGR. Chris himself was recommending a mount slightly negative of Team for normal guys (on the older models) - not recommending that everyone non-pro, non-trickster just bag it and ride a S7. But Spindrift and I actually agree, slight adjustments either way are a matter of taste - as long it is within a reasonable range and maintains the ski's character. That's why I think that the mount point shouldn't be determinative in this case if it was done correctly.
Ultimately, I don't think that you need to become the skier that Pollard is to enjoy his pro model, any more than you need to be a peer of Chris Benchetler to ride the Chets, any more than you absolutely need to ride a Schizo or a rental track so you can micro adjust the mount point. But you probably do need to spend some time getting used to the shape, the width and the feel (mount point included). Fix your boot situation and give the season time.
Finally, don't re-mount unless you've given it a shot and really think that is the answer. If the real solution for you is to dump the Opus and pick up something more traditional in feel, one less set of drill holes preserves some resale value.