Yes, Jumpgates... Back in the 90s, someone did a semi-official Q&A document where many FS concepts were discussed. I can't remember what it was called, and I don't know if it's around any more. However, the following is my perception of things Jumpgate-y.
Although a commonality exist, each Jumpgate is different, both in construction, size, appearance, orbit, and function. Basically, each one is a unique object of marvel and mystique. By mythological analogy, there are many magical swords in our myths and legends, each one different and only one is Excalibur. It is the same with Jumpgates.
When a ship nears a Jumpgate, the keyholder slots the key into the jumpengine (or a relay apparatus), which then communicates a message to the Jumpgate (using collor and sound codes if I recall correctly).
You need a key and a working jumpengine to make a jump, unless you can manage to calculate jumpcoordinates yourself (which is illegal according to Charioteer laws), or unless you can hitch a ride with someone. When the jumpgate opens, several ships (the exact number has never been specified), can jump through in unison, assuming they all have working jumpengines. Without a jumpengine, a ship (or other object) could be towed through. In all likelyhood, moving an entire fleet from one system to another requires the admiral to divide his forces into several jump-groups, each with their own jumpkey.
So, who has jumpkeys? The short answer is the Charioteers and anyone they have lent keys to. In practice however, many faciton own keys, and some factions have even purchased them from the Charioteers, although the Charioteers still claim technical ownership of keys if only on paper. Then there are rogue keys in the hands of pirates and even legitimate traders and other spacefarers - sometimes you don't want to kiss up to the local Charioteer and tell him where you're going. And then there are Philosopher Stones and the like which has the same effect as jumpkeys, and you also have the Sathraist sorcerers who can magic jumproutes open.
To acquire a jumpkey, the easiest would be to hire a Charioteer with the right sett of keys, either as a permanent member of the crew, or as a Jumpnavigator for the journey. Getting hold on your own keys as a non-Charioteer should AT LEAST be a drama in and off itself.
At a busy jumpgate, there is likely to be stationed ships representing the Universal Church, the local authority, and Charioteers Guild interests. While you wait for your turn to jump, your ship might be boarded for inspection by any of these, or other parties who can muscle it. You can always complain to your boss later if you feel tread on, but she is unlikely to go out of her way to heal your sore ego over a bad inspection that took place weeks ago, so there is little a free trader or other small fish can do to resist inspections. You COULD lock the doors and train your guns at the would be inspectors, which is a guaranteed way to insure you're life's going to be interesting from here on in.
At busy jumpgates, there will often be a queue of ships/group of ships waiting to jump. The ranking Charioteer will typically device a priority list, except that Imperial forces on official errends always goes first (its good to be the Emperor's lackey). If you want to jump with a group further up the queue, you could contact their leader and offer a bribe to hitch a ride, but they will probably demand to inspect your ship to make sure they do not aid and abet some criminals.
As you indicate, there could certainly be Charioteers who set up shop at busy Jumpgates and rent their services to key people through the gate. I like that.
Calculating jumprout cordinates yourself is very difficult, very frowned upon by the Charioteers, and it must be done on-site as a back-and-forth trial and error to coerce the jumpgate to play ball. You cannot use the same coordinates forever, as everything moves in physical space (and probably more in whatever space the jump between gates takes place). The jumpkey somehow "ties down" a sett of coordinates, so that it can be used for some time. However, due to "data drift", even jumpkeys will stop working at some point, which is when you'll crawl back to your friendly Charioteers office and try convince them that you've been a good boy and deserve to get a replacement key. (There has even been speculations that the data drift thing MIGHT be a Charioteer fabrication to ensure a monopoly on jumpkeys, although it has never been officialy stated in canon, I think. In any case, Philosopher Stones etc. with jumpkey powers never suffer "data drift").
Finally, barbarians, the Vau, and other aliens or Lost World folks all use Jumpkeys (they were invented in the First Republic/Diaspora times, and have been with us ever since). Exactly how they get their hands on working jumpkeys is unknown in canon, but presumably they have ways of manufacturing jumpkeys themselves (which would be an incentive for the Charioteers to lobby against foreign contacts). The calculation of on-the-spot jumps is probably also a common way for barbarians and the like to get around.
Did I answer all the questions here, I wonder?
