G3/G2 and G42/G41 ?'s

OK lets start with circular interpolation. I hear that it is better to use IJK than end point and radius. So I generate my G code off of CAD R12 by setting the same origin as on my workpiece and getting my points from that. Say I was working in G90, then I and J would just be the value they are in CAD? Say I was working in G91 then they would be the DIST between the arc's center and start point ( that command shows delta X and delta Y)? Am I correct so far?

Now in terms of cutter comp. I understand that I have to have a lead in move and at the end of that line of G code it would be good to put in my G41 or 42 command. It is also important to have that lead in move in the same general direction as teh cutter first move in cutter comp. Now say that a cut is finished and I want to go back just basically in the opposite direction. Do I need to have a new lead in move because cutter comp will be changing from L to R? what if I want to make the cutter come into the workpiece by changing D, will that work without command a tool change? I was thinking of programming the coordinates of teh profile I want to cut and then just changing direction and also changing D to a smaller number w/ every pass. Is this doable? Anything I should know?
Thanks for all the help, this is my 1st CNC