Is Shimano the only company with a sealed ratchet mechanism in their freehubs?
Do any other manufacturers use internal ratchets in their freehubs?
Many other companies manufacture freehubs with internal ratchets, both clones that are Shimano compatible and variations of their own. Specific branded examples include the Giant FH06 freehub or the Bontrager Select freehub, but like most hubs and freehubs these are likely OEM and sourced from Taiwanese manufacturers such as Joytech
Shimano did not invent but was/is the patent holder for that design of freehub. Trying to find the specific patent is hard as Shimano has many many patents even just relating to hubs, the closest example found was this one. The word Freehub is also a Shimano registered trademark. There is more information and references on this wikipedia entry
Although unable to find specific data to verify it, the sealed internal ratchet design freehubs are usually relatively heavy, especially compared to the exposed ratchet designs, and you tend to find higher spec/higher priced hubs use the exposed ratchet design. The sealed internal design is also usually a consumable item, you just replace the entire freehub when required rather than servicing it as you can with the exposed ratchet designs.
What is the largest diameter hole in such freehub cassette bodies for through axles
I remember, so not conclusive in any way, having to get hold of a large (19 or 20mm) Allen key to remove a freehub from a hub that used an oversized axle.