I don't know how to answer the first part of your question but I can add something to the second.

When milling a head how much you can shave is dependant on your squish. You have to check that out because it's not a perfect science. It differes slightly from engine to engine even though they should be the same. Plus if you are using some different internals then your squish will most likely be different then someone else that is for example using the OEM. I know those CTR pistons have a higher compression so the amount you can mill becomes even less.

How high a compression are you tring to get? I think with those pistons you should already be at 11:1 and anythign above 11.5:1 you can no longer run pump gas.

If you deck the block then yes you do change the base timing.