Hello from the USA.  I own a M2 half track.  You've made a good catch on the matter of the rear idler alignment.  Very observant.  Taking those cone washers out is simple.  Get a mallet and give the axle flange a good firm hit.  The shock will cause them to become loose.  It seems counter intuitive but that's how its done.  
First, a couple of things regarding maintenance.  All the threaded bits that provide tension, or align the rear idler and the stops that limit the idler arm; those are best kept in good order by coating them with preservative and wrapping them with tape after final adjustments.  That's how the U.S. Army did it and Ordnance Maintenance, no doubt, knew what they were doing.  There is no job quite as awful as trying to adjust those when they've been allowed to get dirty, caked with mud, corroded and all the other terrible things that happen to long threaded rods that live in that area of the vehicle.  
Now, assuming your idler bearings and races are the correct ones for the application and assuming your idler hub is healthy, you can pretty much rule that out as contributing to misalignment.  I suggest you disassemble the idler assembly entirely and look at bearings and races closely.  It is worth doing because it is rare that a half track is disassembled to a degree that it is easy to look them over so, now is the time.  Those bearings been subjected to a lot of heavy lateral loads if the idler is that far out of true.  It's a possible cause and also a possible victim of improper idler adjustment
There is an aligning strut that runs from the rear frame horns at roughly a 45 degree angle, and over to the idler control arm assembly.  It has a large adjusting nut centrally located on the threaded bit.  With some careful measurements, you may determine that it has been set incorrectly or has suffered some sort of damage.  
When there is any doubt, refer to the technical manual.  Love the channel.  Good luck!

I don't see a comment on how Good that tank driver is. Going through streets and small towns back and forth from lane to lane. Awesome Driving!

Seeing this many WWII vehicles actually moving instead of sitting behind ropes is what makes these shows special! :eyes-pink-heart-shape:

that convoy is an absolute scene. well done.

The Weald foundation have been legends for years. I’ll bet when you first performed tool abuse on the BARV, you never thought that you’d be transporting their vehicles from your shed. From young lunatics with a stilson, you’ve become slightly older seriously respected in your field. You should all be really proud of what you’ve achieved.

That convoy was special. I couldnt help but get a lump in my throat.  Imagining all those vehicles filled with young men heading to war, not knowing but hoping it wouldnt be their last journey.

1:23:26 I don't know if the chaffee driver has just massive skills and bawls or just luck. But god damn that was close 😰

Looks like when they made the half-tracks, they were assuming they’d last a few weeks, maybe even a few months and then get blown-up. So nobody would be dismantling one, unless high explosives were applied.

59:37, I wish I could see something like this in the USA!!! I'd be happy to be stuck in traffic behind this convoy!!!

I have heard many years ago from an owd lad who drove Scammells that having the sides on the bonnet improved airflow through the radiator.  
Having experience with Gardner engines I have never seen one boil, unless you have really buggered something due to gross stupidity.
I know people like to remove the engine covers to show off the 6LW in all it's glory but on the road use the side panels on the bonnet, the radiator won't be making you drink coffee made with rusty water.

"aw look who it is- youre not important anymore"
the sheer verbal abuse the ever-glorious and regal Ted has to put up with from his underlings is beyond belief

Dad was a Motor T mech in WW2. I was in my teens, training as a car mech in his service station in the '70s, when he would share his experiences from time to time. He once said to me, once you learn how to handle the weights involved in trucks and heavy equipment, you'll never want to go back to cars. I can't watch you pry and beat without remembering him😁Thanks.

Brilliant video as ever 👍 Taking a convoy to the pub with a Tank, seems a logical thing to do 🍻

11 out of 10 for whoever drove that tank on the public roards  His/Her skills at steering on a confined public road were quite extraordinary

Jack  ...  Nice job on the trailer and your clear driver direction are on point as usual .

The convoy through the English countryside makes all the blood, sweat and tears worthwhile.

Props to Jack.  Man he does nice work.  You’ve got some great friends there Mr. Hughes :)

And just imagine, on the build-up to the D-Day landings, convoys like this was a sight seen all over Southern England in June 1944!

I think the star of the show was the puppy. Great convoy

That puppy done cooler stuff than me already