Tiarella, regarding your questions and observations about your winter cheesemaking - here is what Paul Kindstedt in American Farmstead Cheese says about seasonal variations of milk, which speak to your issues:
1. Casein content (%) (and fat - see #2) rises over the season (and lactation), lower in summer and higher in fall (and presumably winter, though here Kindstedt is referencing a study done only over summer and fall). The good thing about this is it makes for a higher cheese yield relative to milk. But, he explains, "large fluctuations in the casein content of milk across seasons...are accompanied by large changes in the buffering capacity [resistance to change in pH] of the milk, being lowest in summer...and peaking in fall. Thus in order to keep the acidification schedule during cheesemaking constant across season, the cheesemaker must adjust for the changes in buffering capacity by adjusting the starter culture usage, generally by using less starter in summer and more in fall" (p. 50). In other words, all else being equal your winter milk will not acidify at the same rate as your summer milk. And acidity, of course, is a factor in the final moisture content of the curd and cheese. So a lower-than-expected acidity of winter milk will also translate to a moister-than-expected curd.
2. The ratio of casein to fat in the milk changes as well. Fat levels also rise in fall and winter, and late lactation, but proportionately more than casein. In the referenced study, Kindstedt notes the summer ration of casein to fat was 0.78 to 1; in fall, the ratio was 0.62 to 1. Then, "This is important because fat interferes with the ability of the curd to shrink and expel whey during cheesemaking, and also tends to reduce the uptake of salt by the curd during salting. Therefore, milk with higher fat content relative to casein tends to produce cheese with higher moisture and lower salt contents, all else held constant" (p.52).
3. Strength of the curd is relative to casein content. Lower levels of casein generally coagulate slower than high levels, and form a weaker curd relative to higher levels (p. 88). Applying this to the flocculation factor method, fall and winter milk with higher casein would being to flocculate sooner than summer milk, and thus reach the desired point of cutting sooner. If going by time rather than floc factor, a fall and winter curd might then be cut so as to favor a higher-moisture curd (it has coagulated longer).
Steps you might take in practice then are a) to pre-ripen the milk for a longer time, to increase acidity and starter activity to a higher level, before coagulating, to compensate for the higher buffering capacity of the higher-casein milk; b) cook the curd for a longer time at a lower temperature (closer to the optimal temperature range for the culture reproduction and acidification) before cooking at the higher temps; c) cook the curd for a longer period of time (increasing acidification and moisture expulsion), and perhaps more or more vigorous stirring.