I know nothing about when officers stopped wearing powdered wigs. I beleive that by the 1790s, wigs went out in favour of natural hair which was "dressed" and then powdered.
The Thorpe memoir referred to is NARRATIVE OF INCIDENTS IN THE EARLY LIFE OF THE LATE SAMUEL THORPE (London, 1854).
I do not know how many wives and children accompanied the troops that sailed directly from Britain to Corunna under Baird. That information can most likely be found in WO 25 at the National Archives of Britain or using ships' muster list for passengers receiving rations only which, if they were on naval ships, would be in Adm 36 to 39 or, on private ships, at the National Maritime Museum in Greenwich. I would suspect that there were 6 wives per company and, as I remarked, for each wife there was usually 1.5 children in my experience so 60 wives and 90 children per battalion.
The statement on wives being left behind at the Walcheren expedition comes from Rifleman Harris of which there are many different editions. Mine happens to be that edited by Christopher Hibbert but I cannot lay my hands on at the moment. I did make some notes from it and looking at them just now I see that he did not say the wives were not permitted on the Walcheren but only that their number was severely curtailed from the Corruna expedition (can't trust memory).
He writes that "the distraction of the poor creatures at parting with their husbands was quite heart-rending; some of them clung to the men so resolutely, that the officers were obliged to give orders to have them separated by force. In fact, even after we were in the boats and justly pushed off, the screaming and howling of their farewells rang in our ears far out at sea."
DE Graves