I took my DT125B for a long ride and had multiple missed shifts going for 5th gear using the clutch. Embarrassing rev to redline. 

It never missed when shifting without the clutch. Is that any kind of clue to you experts as to what the issue might be. 

I did notice that after about 2 hours of uninterrupted riding it seemed that all gear selections got crunchy. Oil level is correct and clutch seems fine with no problems finding neutral with the engine running. So I don't think the clutch is dragging and its not slipping at all.

Bottom line is I simply shift without clutch for the tall gears. But its my only bike in the herd that does this. 

When was your last oil change? I'm a big fan of any time there is a shifting issue, change the oil. Milage is one thing, but such a rough indicator, high miles putting around on pavement vs hard miles on singletrack with lots of dust dust just wear out oil in different ways.

When you pull the clutch the pushrod is going to apply force to the mainshaft towards the right.  

I think that if the mainshaft could move...  and it really shouldn't with the inner basket tight unless the ball bearing is bad or the bearing retainer plate under the clutch is loose...  the movement may tend to disengage the dogs for both 4th and 5th gears.

Anyway, never saw this before so just guessing.  Also, when under power I think the helical clutch teeth might tend to push the mainshaft to the left.

If you decide to check it out while I was under the clutch cover (and probably not finding anything wrong) I'd make sure shift linkage adjustment is correct and maybe tweak the travel a tiny bit to the "up" side so the claw rotates the drum slightly more when up-shifting.  But you need to be careful, too much and it might start acting up downshifting.  
 

Just my random thoughts on 5th gear.
Moving to the right would pull 5th close to the sliding dogs. 4th would go further away.
#7 is 5th but plate #12 would need to be bowed outwards or screws undone to have any baring on 4th or 5th--me thinks.
Anyway, i'd check the drum end above the sprocket under the 3 screw plasic cover incase its tossed a chain & damaged or broken the drum end clean off.
Otherwise i'm out of ideas except i'd suggest you "go in" at least to the first cover but check anything relevant including the eccentric screw incase it's broken from gear lever damage in the forrest.
If nothing then may as well go trhe rest of the way if there grumbling noises going on.
Looks like it has a needle bearing #9 on that shaft too. Only mentioning in relation to grumbling noises!!.
Sorry i see you said Crunchy noises so did you mean resisting engagment!!.
Gear #8 is pressed on & keeps 5th gear in place. So if #8 moved away it'd have an affect on 5th engagement.
That's very unikely & not much skope to move sideways.
Also if it moved it'd likely spin then you'd have no second gear--without looking at pics again.
photos.app.goo.gl/XAcFpfVSRaErwZ1F8

What everyone else said plus:
1.Check that the shift lever is tight on the shift shaft with no up/down or side to side play. Needs to be perfectly tight! This can affect shifting.
2.When you shift into fifth, hold the foot shift lever up until you let out the clutch and get on the gas. If this works it is just masking the real problem(s). But I have ridden bikes this way.
3.If you pull the clutch cover off make sure the spring holding down the shift drum roller is good, the plate that holds the pins in the shift drum is tight and not bent, and the pins that the claw engages are not loose/bent. I have a feeling that when fifth is missed, the roller isn't down all the way between pins on the shift drum.

I always hated the little plastic cover that the chain would take out when we couldn't afford new chain and sprockets! On the 125 they kept that a few years later than the 100 and 175.
Good luck!

Once again you guys have provided me with an awesome list of possibilities and I thank you. I will most likely dive into this during off season winter months which are right around the corner. It’s a chilly 45F this morning! The only problem with that plan is doing test rides…..

In the meantime, it never misses when using a crisp shift without the clutch.