Eureka!

Sometimes the simplest solution is the best.

As I mentioned, I changed the blanket pattern a little bit in an attempt to avoid an eternity of piecing together and sewing in loose ends.  Keeping the non-active yarn to one side is not really that hard.  I've gotten used to turning my work in just the right way and rotating my working balls with every colour change so that nothing is tangled and everyone is happy.

But simple changes are sometimes no so simple after all.
I've been bringing my yarn forward before slipping the last stitch of each side so that the edges are very easy to pick up and make a nice looking seam.  This technique is working really well... for all of the "normal" squares that don't deviate from the pattern.

The problem is that when you don't want to cut the yarn, you have to begin using it at the TOP of the square, where the first row (and all odd numbered rows) would end, not the bottom where the beginning of the odd rows are.  Since the colours alternate every two rows, the odd rows are where the new colour should begin.  It is also the row that the centred double decrease happens, to give a nice balanced 90 degree angle in the centre of the square.

the "yellow" line on the bottom and the "green" line on top

You can see here, the yellow line's squares all begin with yellow first, then the "accent" colour comes next.  Most of those accent colours are still attached to the work because I didn't want to cut them.  In the "green" line, all the squares that come after (or to the left of) the solid green square will have green as the first or primary colour.  This is where the trouble starts because it's where I first encounter the uncut yarn.

OK, so let's say I start the green and turquoise square.  I pick up and knit 24 stitches from the solid green square with green, pick up and knit 25 stitches off the side of the yellow and turquoise square, turn the work and knit all the way back in green.  Great!  But now, instead of picking up the turquoise yarn and knitting to the centre, doing the decrease, knitting some more, and slipping the last stitch like I am supposed to... I have to start on the other end.

This is OK, no big deal.  I just switch to reverse garter which is just purling all the time instead of knitting all the time.  I just knit all the green rows and purl all the turquoise rows.  Problem solved.  Except when I have to decrease in the turquoise odd row, I'm working on the "wrong side" and I can't just do the CCD as directed.  It's going to look wrong.  Don't worry, I'm clever.  I did this:

"This" is picking up two stitches at once through the far back loop, slipping them,
purling the next stitch, and passing the two stitches over

problem solved.

Except when I'm alternating knitting and purling every two rows it's messing up the whole slipping the last stitch thing.  It's not wrong, but it's not right either.  It looks like this:

The green side is even, the turquoise side is lopsided.

On this edge the aqua is smaller and on the other edge the green was smaller.

It's not terrible, and I make it work for the rest of the square, but when I pick up the stitches for the next square it looks like this:

Do you see how there are little nubbins of turquoise in the vertical crease?  Those are evil little nubbins of death.  All the other creases are smooth little ditches of happiness.  Why can't these nubbins get with the programme?

So on the next square, I try to mix things up, do a little experimentation, a little je ne sais quoi.  No, seriously.  I don't know what I did.  Just look at this mess:

The dark blue and green square is how it's supposed to look.
Try not to look too hard at the aqua one.  You might get hurt.

This is how the back looks when you pick up stitches off of it.
The front, if you look at the picture above, is not so bad.  But it's not great either.

I take it back little turquoise nubbins.  You are adorable and cute.  Aqua edge... you are a monstrosity.  I am sorry.  I still love you, and I will keep you.  You're part of the blanket now but... I will never repeat you.

Fortunately by the end of the aqua square I get an idea to always knit the knit side and always purl the purl side, but still do the slip thing and make sure I keep the yarn strands on the inside of the edge stitch... except... didn't I try that already?  a few rows ago?  No, that was something else... I think?

Oh boy, it doesn't matter anyway, because I accidentally cut the dark blue from the last line so this square will be normal!  yay!

By the time I got down to starting the turquoise line I was very nervous.  There it was. The yellow strand of yarn, mocking me from the top of the previous square.

And then in dawned on me.

Why not just work the entire square in reverse garter?

I don't know why I didn't think of it earlier, and I'm sure I'm missing something, but I feel like a dumb dumb and a genius all at once.  
So, yay me?