Thanks as always. Three points in explication by return post:
* Lamarque post-Wagram. Apologies for differing uses of the word "detach." Without getting into the theology of this and similar words in current US military terminology (which can descend into absurd casuistries), simply meant that no units (other than the two wandering 92e battalions) were permanently removed from Lamarque's command after the battle. Owing to the heavy casualties suffered, the entire Army of Italy was reorganized post-Wagram, the key feature of which being the dissolution of Seras's division and distribution of his former subordinate regts/battalions to other divisions (as outlined previously). As part of this administrative action, the two battalions of the 92e returned to Broussier. As you correctly note, Lamarque was ordered to send ("detach") several battalions to establish/occupy a bridgehead after the battle, but from an administrative/organizational standpoint, this was a temporary measure; those units remained part of his division (while the two/92e battalions did not).
* Formation choices. Will have to give this more thought, but the great frustration for those of us interested in such things, is the absence of solid information on how MacD's large formation changed or did not change as he advanced, had to face new tactical contingencies and suffered fairly appalling losses. Broussier, for example, states that he formed his division into a "carré simple" at least twice during the advance. Sadly, as mentioned earlier, many/most subsequent accounts do not have any better sourcing than what is present in MacD'S after action report and Broussier's journal.
* Broussier's journal. The text presented in Koch/Massena pp. 415-17, is Broussier's "Journal historique de la Division Broussier, pendant la Campagne de 1809. A l'Armée d'Italie du 10 Avril au 12 Juillet" (MS in C4/10, Archives de la Guerre). Believe it has also been published in whole/part elsewhere? Without going through a word-by-word text check, it is interesting to note that the transcribers who put this in Koch's volume dropped the word "deux" in ref to the 92e at one point. It should read: "...le 9e Régiment, avec une bataillon du 84e fut déployé en ordre de bataille, les deux autres bataillons du 84e et deux du 92e furent formées en colonne par division...."
Best from (now sunny and warm) Virginia.