Q1 Yes Ethernet is much more reliable than USB and Yes while it's possible to connect thru a hub it's not recommended so the honest answer YES but Don't.
Are there better options than the Uc100 then yes but they will cost more money. My personal Pref comes from Cslabs but not cheap so the next best would be ESS connected to a Good BOB like PMDX126.
After that then you have Pokeys 57Cnc which is an excellent little board packed with features for not a lot of money. Again needs connecting to a good BOB.
The BOB is critical to a stable machine.

Q2 Yes it's possible provided you have correct grounding and cables. Also Good idea to run the VFD on it's own supply and fit RFI filter.

Q3 No issues and RFi filter is nice but not required in many cases, thou it would be a good idea to fit one because of the VFD esp if sharing same mains line.

Q4 Yes it's worth it and not difficult. Two ways to go about it. You can either just have Mach turn spindle ON/OFF which is most usefull. Or ON/OFF plus control the speed which is nice to have but not has useful as ON/OFF and depends on the Jobs your doing to how useful.

Q5 Yes it's possible but not easy and you'd have be messing around with settings you really shouldn't be if don't know what your doing.
Setting up is easy enough and just involves setting a Few parameters, takes 2mins. You'll find all you need here in this thread.
http://www.mycncuk.com/threads/5814-...ce-More/page14

One last thing you mention running the motors with 36V.? For a good router you'll want more than 36v to give you speed. I wouldn't run a router much under 50v with 65-70Vdc being normal with 3nm motors. Obviously you'll need drives to match.!