You’re hopefully going to get several answers, as there are seldom a single, correct answer to these types of questions, but here’s my 2 cents:
1: If you buy Rhino, you get the whole package, which means all the modelling tools (NURBS and mesh tools), Grasshopper (visual programming for modelling), the Rhino render (a slightly out-dated render engine still capable of doing some OK-but-not-great renders) and Cycles render (a real-time raytrace render, which is more modern - if your graphics card and/or CPU is up to the task). What you don’t get (for free, that is) is some of the more complicated plug-ins and render options. There are hundreds of different plugins, some very general, some very specific. Some (a LOT) are free, some not. Most of them can be found at www.food4.rhino.com, where there’s also a lot of plugins for Grassshopper.
2: That really depends on what business you are in. I’m a product designer, so for me it’s mostly SolidWorks, Inventor, NX and similar mechanical design programs, both upstream and downstream; meaning where we get data from (clients and suppliers) and send data to (clients and manufacturers). We also do quite a bit of architecture, where most of our data come from ArchiCAD, gets detailed and set-up in Rhino and then we use Keyshot for rendering. Others will have different work flows, so it really is a matter of which industry you are in.
3: Are we talking apps like Duet or similar - where the iPad is tethered to your PC or Mac via cable and can then be used as a secondary monitor/digitizer? I’ve never tried it, but I guess it would work. I know there are some users here on Discourse that use Wacom pads as their primary Rhino-input, which can be of use if you suffer from carpal tunnel syndrome or similar. Other than that, most of the input in Rhino is more precise - at least in my line of work - and therefore better suited for mouse input. Freehand sketching is primarily done by hand or using Procreate or Sketchbook on the iPad.
That’s our current work flow, but as mentioned, there are probably almost as many work flows as there are Rhino users. It really depends a lot on which industry you are in and personal preference.