Plans for running new 8/3 NM-B service for oven
OK here we go.

1) There is no violation to splice the conductors as long as it is "mechanically and electrically sound". Just remember that the splice is where things can go wrong, and they must be done in a junction box and accessible for maintenance.

2) It is alright to use a junction box for a transition. Just remember that when you are bring the romex out of the box with no conduit, the opening must be bushed. You can use a plastic snap in or a connector or a chase nipple with a lock nut.

3) In this case your are really not using the EMT as a conduit per say, but as a chase for the romex and to prevent damage to it. You can also buy pre manufactured 90's if you need to use them rather than expensing out a bender, and fitting them may be easier for the non skilled. See my remarks on bushing in 2). Also see your own remarks on the 1 1/2 in 2) as far as nail plates go. I prefer the overuse of them than take chances. 

4) It's always a good idea to match the breaker with the Panel manufacturer although other manufacturers breakers will match up in certain panels.

5) There is no code requirement to use an AFCI with a range circuit.

Hope this helps, good luck.