u/othelloblack

Its 450 yards from the cross roads behind the sand pit to the three way intersection thats now just north of the current Lion Mound. Halkett's location is usually centered on that three way intersection. The map from the Waterloo Assoc. that one labeled Second Attack shows a gap of 300 yards from left flank of Halkett's 5th brigade to the right flank of Ompteda's brigade. And shows both Kruse and Detmer inside there represented as 100m wide blocks. 

https://www.waterlooassociation.org.uk/2018/05/31/attack-of-the-imperial-guard/

then later in the Third Attack map they have Detmers in 6 columns, taking up some 250 yards. These are battalion columns and if following standard practice, some 40+ men wide and about 12 men deep. So perhaps a 50 yard frontage realistically and you need gaps between the columns too; for horse artillery, heavy cavalry, skirmishers, scouts, guys running under cannons, etc.  That just doesnt seem like enough space to hold all that.

For example the 5 battalions of Middle guard that are attacking this area are about the same size as Detmer's 3100 man brigade and they are possibly 5 columns, but more like 3 in mixte order formation and one or two others in column. And these take up almost that entire 450 yard corridor as shown in the maps.

almost none of the officers in Waterloo Letters that are part of Siborne's legacy mentioned Detmer's role at the climax of the charge although several of them in Halkett's 5th brigade  do recall the Brunswickers being in rear and to the left of Colin Halkett's brigade; identifying them by white insignia (crossbones) on their Shakos.

It seems accepted that Kruse's brigade was driven back leaving Brunswick's indep division between Ompteda on their left and Halkett on their rights. Its possible both these Halkett's 5th and Brunswick were driven back somewhat

Even though the Waterloo Letters are maligned, at least one British officer in the Waterloo Letters did see them: 
Lt COl Taylor of the 10th Hussars in Vivian's brigade must have seen both the Brunswickers driven back and the advance of Detmers:  on p172 of the Waterloo Letters:

" ...they rather fell back upon us and one battalion of Brunswick Oels in close columns retiring being in want of ammunition, I believe, but in good order was now passing through us between my squadron and Major HOwards. Just then two or three battalions to the left of it advanced in double quick their drums rolling and drove back the enemy.." 

Interesting that he mentions the double quick pace because Chasse himself recalls ordering  the Storm Cadence and others mention that pace as well; presumably a double quick step. 

Two or three battalions in column makes a lot more sense given the spacing involved

waterloo letters can be seen here:

https://archive.org/details/waterlooletterss00sibo/page/150/mode/2up

At least one officer from Halkett's brigade did see the Dutch attack. Im not sure the original source this quote from a prior thread on reddit:

British eyewitness (captain Edward Macready, 2/30th regiment of Foot), wrote: "The Dutch troops advanced in a state of high excitement, cheering wildly and lifting their shakos on their bayonets." 

Macready fought in the left most (comprising 30th and 79th regiments) of the two giant squares that Halket's brigade had been forming earlier. Halkett's battalions are at the time of the Guard attack apparently going into a line formation from square but like a lot of things no one is real sure when and if. One of the main issues is was Halkett driven back, which Halkett says no except this one time he had to pull back the 69th regimenet IIRC, but then Halkett was wounded around 630 pm and probably did not witness the firefight/highwater mark of the Guard.

So I dont doubt Detmers was there facing the MIddle Guard that lead off the attack. but I question the 6 column wide formation doesnt seem to fit. It is reported that CHasse had the brigade march to their place in front with 3 columns across. Then later when he realized the danger he went back for the other 3 brigades also in column. These accounts then claim he went in 6 brigade columns across. I think it more likely that there were two waves of 3 columns each. And these columns would have maneuvered around clumps of stragglers as they move forward possibly through remants of Halkett's brigade and the Brunswick division.