(4) and (3) Yes, it is available for general public. And as @Sam_Norris mentioned, it is advised to contact a local Peplink Partner to find a suitable solution. https://www.peplink.com/peplink-certified-partners/
(1) Peplink firmware has a huge amount of features and many hidden, but no need to worry about it. A good place to start with configuration is using @Michael234’s checklists on Pepwave Surf SOHO router initial configuration - RouterSecurity.org . Also described in this tutorial is to save your configuration files each time you adjust some features so you can return to satisfying configuration whenever needed. Or do a factory reset and reapply the last know configuration you had.
(2) and (6) There is a whole range of Peplink devices with different features. Or you discuss this with your local Peplink Partner or you can write down some necessary features before going through all the comparison matrices. As you described, that would be ‘Multi Cellular’ to have more bandwith and ‘Wifi’ for all the devices if they support. Or is there some need for ‘LAN ports’. When you are on camping sites or at gas stations, you can also take advantage of the local Wifi spots by using the ‘Wifi as WAN’ feature. And as mentioned by the others, indeed the Max Transit Duo seems very suitable. A last situation in a RV that I saw passing by on the forum is if it’s a metal trailer or not, to think about how to place the router and antenna’s. Surf soho Antenna splitter? - #5 by jharrell
(5) It is adviced to use separate cell providers. I think @Joey_van_der_Gaag explained best in Live Event Streaming Setup - #2 by Joey_van_der_Gaag. There are different options to use multiple WAN or sims, but, using info from @MartinLangmaid, Bonding is a point to point technology. More info here: SIM card Bonding - #11 by MartinLangmaid. You can use multiple WAN without real bonding and this is worth reading on: Confusion over speedfusion and traffic control algrithums - #2 by MartinLangmaid. If there is a need for Speedfusion or not, you can discuss with the local Peplink Reseller also.
When searching for ‘wifi rv’ on the forum, there have been multiple user mentioning RV solutions and situations.