In what way and to what extent would the haemonculi be used?

Answering the question on alchemy, it will play a role in the RP, but typical alchemy is considered to be related with normal cooking. In the same way a good meal makes one feel good, so to does that one relaxing elixir, to illustrate the point. Both affect subjects when ingested, what makes them different, besides the mere ingredients or the occasional spatter of incantations? Therefore, most witches disregard the magic of alchemical creations and treat them with relative normalcy.

That said, alchemy is still an area of skill and possible expertise, just as cookery. They aren't ignorant of it or its capabilities (similar to how no one eats poisonous mushrooms, no will will ingest a random poison willy-nilly; they understand), and in fact, some witches (especially those whose skills and interests are within the field of alchemy) still use the field to achieve various effects, but it's not seen as anything utterly spectacular in general. Now, specific elixirs and potions may be more greatly prized, but as with certain creatures, these are the exceptions and not the status quo. Thus, if you're planning an alchemical character, have at it, and don't let me change your mind on anything. A traditional alchemist, one who uses magic and concoctions to achieve effects is still viable and possible, just expect the character to be reacted to a bit differently. If you want to try something a bit different though, you could have an alchemist also have a role as a groundskeeper or cook, which both share certain areas with typical alchemy.

One thing to keep in mind though is the technological resources available (or should I say unavailable) in the realm. As detailed in the first post, the age is reminiscent of the dark ages/middle ages, things like automatons and alchemical/technological marvels and creations will be hardly existent. And those that are will be more-or-less simple and far from advanced automatons, although not technically useless. Technology, even with the help of magic, just isn't at the proper level to be sufficient for the more 'magitek' like creations.