How (Not) to Flatten a Piece of Wood

I want to glue the fretboard to the neck, but I want the neck to be very flat and true first.  The fact is, however, this board is not flat.  The fiberglass/epoxy layers are probably more compressed in some places than others, the basswood probably wasn't exactly 1/16" thick everywhere - this is how it goes.

Near the start of this in a post entitled "Disaster...Or Victory?", you can see that I set up my laminate on a sheet of 1/4" plate glass.  This very typical and common type of glass is sometimes called float glass for its habit of being created on a pond of molten tin.  Molten tin!  So it's generally considered pretty flat.  The idea was, lay the laminate up on a perfectly flat sheet, weight it down against that sheet, and you get a board with at least one perfectly flat face!

Well that didn't work out so good.  Somehow it didn't happen, some entity or phenomenon got between my float glass and my laminate, and it just didn't work out that way.  So I have to flatten this board.  I'm naive so I figured I could just rub the board against some sandpaper affixed to that very same piece of glass, and make it flat.  But only the actual neck part - the headstock is going to get rebated away anyhow.  First mistake:  I should have taken the headstock down first by sawing, to get it out of the way.

For a while I had the glass on a carpet and pushed down on the board, as I rubbed it around in circles.  I drew in pencil on the board to see where I was taking off material - where the high spots were.  After about 40 minutes at 120 grit, I did get to a point where the sandpaper had cleaned up the whole board, but all was not perfect.  There was still a low spot in the middle of the neck!  I hypothesize that by pushing down on the piece, I flexed it into contact with the glass, thus taking more off the part directly under my hand.  In some strange way, the high parts escaped.  Perhaps, also, the glass was flexing into the carpet.  One thing I did right:  I constantly swept the sandpaper and the board clean to prevent little piles of dust from glomming onto the paper, forcing the wood up over it.  Here's the board with pencil writing on it:

But it was much better than before, clearly.  I was still hopeful.  Time to adjust my attack.  This time I placed the glass on a long level and gave it some new sandpaper, still 120 grit.  And - here's my innovation - I didn't press down on the board, I just pushed it gently from either end, back and forth.  And I checked it with a straight edge regularly.  And lo! it got even better.  But all was still not perfect, in fact I had a totally new problem:

I don't know if you can see it here, but the left side is thicker than the right.  The top may be flat, but the whole thing is canted over.  My cheap but cool digital micrometer is AWOL due to a battery condition right now, so I can't tell you exactly how much.  1/3 of a millimeter?  Something like that.  Is this a problem?  Well, two things:

1. The damn thing's still not straight enough.  I want it about as perfect as possible, because if the fretboard goes on flat then in theory I will have to spend less time leveling the frets.
2. I don't want it tilted over.  That irritates me.  I'm building a simple guitar, not some fancy ergonomic thing.

I still have enough wood to stop and figure out what to do.  Fender neck blanks are sold 13/16" thick, and this one is about 15/16".  The only trick is I don't want to stop on an epoxy/glass layer, or partway through one, as that'll force me to change my choice of glue from Titebond to epoxy.  Big deal?  Dunno.  And I'm not sure many machines will be happy cutting through a nasty layer of garbage like that.

In any case, I'm reaching out for help here while I've still got room to move.  I'll post more when I get the guidance I need.  You'd be surprised how hard it is to find answers to this on the internet.  You're either supposed to use a giant plane with ninja woodworking technique, or you're supposed to have an electric thickness planer.  Even to use the planer, I'd have to prop up the other side of the board (which is even worse than this one) artfully in a facsimile of a flat surface to give the planer something to push against.  I suspect I'm going to end up with a belt sander and a sanding frame, or some homemade router rig set up on a table saw table.

I have discovered this:  the concept of flatness is a human one.  It is a state known only to water on the calmest of days.  It's a strange place to be, knowing exactly what I think I want in a very geometric way, and being unable to arrive at that configuration.

I am a monkey.