That's actually a series of questions & not as straight forward as it may at first seem. The wartime contract with Holland & Holland did not cease till April 1946, so as you say rifles were being set up until then.
I don't know of anyone who can say unequivocally whether or not post war dated rifles (ie 1946) were ever set up, as none of us have seen or at least had access to the serial numbers of every No4 T ever set up. In my personal experience of a fair number of 4T's & L42's I have never seen a rifle dated later than 1945, but in amongst the list of L42 conversion serial numbers that Simon & one or two others have are one or two very late BSA serials that look to me as though they could represent possible 1946 rifles (Y prefix). But, one cannot be sure from serials as I suppose the Y range could have commenced in late 1945; I certainly can't say for sure. Maybe someone else can.
Your second point over whether earlier rifles were converted at the end (the scopeless Savage & BSA rifles are often mentioned) is also a difficult one. I think the general opinion seems to be that some early rifles were partially converted late on. I have always had some reservations about this subject & I don't want to muddy the waters too much but there are minor variations in the shape of 4T front body pads that studying them over the years reveals; the front pad on a 1945 rifle will be subtly but most definitely different in several minor respects to one off say a 1943 rifle. Having seen quite a few Savage Mk1* 'less T' rifles over the years, their front body pads have invariably been of the 'early' type not the 'later'. But, I suppose pads could have been fitted but not final machined early on, or I could just be talking a load of XXXX! But as I said, the official view is that some BSA rifles (the ones I've heard of have mostly been of 1944 mfr) & a quantity of Savage rifles were completed then, minus their scopes. I have never heard of earlier rifles being fully completed with Mk3 scopes in 1945/46, so I would doubt that you are likely to see a 'genuinely originally matching' 1941/1942/1943 rifle with a Mk3 scope.
Having stated the above, one has to differentiate between 1943 (& earlier rifles) & 1944 rifles. In my experience 1944 rifles may commonly be found with Mk3 scopes (the Mk3 went into production in the Autumn of 1944). However, they can also be found genuinely matching with Mk1, Mk2 & Mk2/1 scopes as well. The Mk2/1 appeared in 1944 & the Mk2 was produced through most of 1944 too. There is also the oddity that the scope manufacturer William Watson was bombed by the Luftwaffe & introduction of the Mk2 scope was seriously delayed, hence 1944 dated Mk1 scopes produced by Watson are not uncommon. It therefore follows that one might plausibly expect to see one of these Mk1's on a 1944 dated rifle, though I've not owned one as yet.
Does this help or have I confused you more?
ATB