The word 'knight' has a whole bunch of different definitions attached to it. Originally, the words that preceded it just meant 'professional warrior' and then it became associated with the most dominant type of professional warrior, the heavily armoured person on horseback, which eventually became an official noble title - equivalent to the Bretonnian Knight. About the same time it also became associated with the Feudal system, where those professional warriors were granted land in exchange for their service and to pay for their equipment, but over time many 'knights' started to pay their dues in taxes rather than combat - equivalent to Mathilde's title of Knight. During the Crusades there arose various military orders empowered by the Catholic church instead of a monarch - sort of equivalent to the Empire's Knightly Orders associated with Cults. After this period, a variety of people founded chivalric orders of all sorts inspired by the earlier military orders, some noble, some religious, and some neither, and from this point on someone being a 'knight' doesn't really tell you anything about who they are or what they do. This is where the Empire's secular Knighty Orders come in.
The Holy Order of the Templars of Sigmar are a Holy Order empowered sort of by the Cult of Sigmar and sort of by the Emperor, so it is not technically incorrect for them to call themselves knights, but it probably would get them side-eyed by all the guys who actually do ride around on horses in heavy armour.
Originally, 'chamberlain' just meant a servant that tended to a noble in their bedchambers, but it quickly became noticed that this person ended up having a lot of influence over said noble, so people who wanted that influence started to want to be a chamberlain. So the title started being given as a sign of favour and trust, rather than just because the person knows how to make a bed properly. And of course these are important people that don't actually want to spend their time making beds, they just want to have the noble's ear, so they delegate the actual work to a servant and over time 'chamberlain' becomes completely divorced from the original meaning - it just means 'a person who the noble trusts'. From there it splits into a bunch of largely unrelated roles - a chamberlain might be the person that manages a noble's household, or it might be a person who collects rent in a specific area, or it could be a fancy title for any economical position, or an entirely honorary title that actually has no duties or privileges associated with it at all. The position of 'Chamberlain' in the Witch Hunters has probably been on a similar journey, where it was originally an assistant to the Lord Protector that was eventually made an official part of the chain of command.
That title means that the person has the authority to sanction people to act as Witch Hunters, basically like a sheriff forming a posse. So he'd have a separate title within the hierarchy of the Templars.
