Thanks everyone for sharing and advice. Before rehoisting the mount topic, I want to emphasize a valuable lesson.... PLAN YOUR VIEWING SIGHT LINES BEFORE DRILLING HOLES FOR YOUR LCD TV MOUNT. The mast support is going to most likely block one side or the other.
Now, I agree that those 12mm screws into the back of the TV seem the weakest link of the system. But again, it's only 17.5lbs. BTW, the TV that I'm currently trying is a
Vizio VA26, but I may change this...
Don at Hunter wrote me back saying that they'd never installed a wall mount TV into this boat ('88 Legend 37), but that he'd suggest through bolts. I will stick with my 1" lag bolts; If I hit a wave that hard, I think that one of the other points will fail.
Marine LCD Mount Points of failure:
Wall Bracket to Wall
Hanging Bracket to Wall Bracket
TV to Hanging Bracket
Poor planning 
While I am dismayed that I now have these deep holes in my bulkhead, I feel pretty good about this mount. Last night I finished hanging the TV and tightening the security screws and I lay beneath the unit on the settee and wrestled it back and forth, and up and down, listening and watching closely for slippage. I'll do more rigorous testing before heading to Catalina.
There will be more photos to come but this one shows the wall bracket and one of the hanging brackets (not attached to TV). I've circled the lag bolts (5/16" x 1") in yellow on the wall bracket. On the hanging bracket, I've circled the security thumb screw in magenta, and in cyan I've circled the extra vertical hole which I drilled to vertically center the hanging bracket on my TV set.

Just to the outside of each hanging bracket, I am considering a "lateral stopper" solution: either a protruding lag bolt, or a block of wood screwed into the bulkhead.
By the way, this is a very inexpensive mount. It is the
K2 TB, bought from Costco for under $50. It is very well made though, and comes supplied with a plethora of screws, bolts and washers for every imaginable unit or mounting style. And, customer service was very prompt and professional in responding to my inquiries.