if you ask me: I would strand the yarn over if it is for a few stitches only.
And if the color moves by a stitch to 2 in the next row: yes, the same bobbin.
On the other hand: that spot that you talk about is that light gray line along the collar and it will move further and further away from the other light gray part. So I might consider a seperate strand for that, that just moves up that edge all the way.
Remember to wrap the yarns around each other with every change so that you have no holes in the end.
I think if I were doing this project (well, I am not, but it looks nice), the I would print a seperate copy of the chart and use some different color felt pens or highlighters to color my way through it, just for reference: where will the same bobbin be used and where will there be a new one used. Then you can also see how many stitches you will make with each bobbin and you can estimate the lenght of yarn to roll onto that bobbin.
I think I would test-knit a swatch, the mark my thread, unravel a counted number of stitches and measure the length that I pulled out. Then you know how much yarn per stitch you use. To that I would add the acutal cm / inches of hight the bobbin will be carried for, plus, let's say 20 % for the crossing part, or if the outer line will run very diagonal: more than 20 %.
Then I would do an extra 20-30 % surcharge on every length of yarn and add another 10 inches as tail end.
Well, my intarsia experience is not quiete so elaborate but I would hate to have to start new bobbins for the same color all the time and I guess you have to assume that there will be some yarn thrown out.
That blanket seems like quite the project! But it will be awesome when it is done!
Where did you find the charts?