First, not liking halyards lead aft is a personal preference that not many people share. I particularly like my main halyard at the mast because that's where the reef lines and the tack are. I can reef without running back and forth. Also it's closer to the bow cleat to blow the main halyard when sailing onto the mooring. As for the Jib halyard, we have roller furling so having that big coil of line hanging around in the cockpit all summer is just in the way. Spin hals - I'm at the mast anyway, why do I want to go back to the cockpit to hoist. So, that's just my thoughts, I'm usually outvoted on this one. My personal opinion is that hals to the cockpit work OK when you have full crew but if you have short crew (we double hand race) and you have to be at the mast anyway, why not have everything in one place.
But as for internal halyards. To get in at the top, no problem, they just lead right in. Since I still go to the mast winches my exits are above there, staggered so as to not create a weak sectoin of mast. When I lead mine through the mast, the mast was brand new so I didn't have to get around the spreader bar, wiring bundle spin track screws, etc. but I think your plan will probably work OK.
And to turning blocks. I avoid drilling a hole in my boat in any way I can, especially one that can leak and/or cause wet decks. Of course it can be sealed so that this is not an issue but no hole at all is less to worry about. Beyond that, you will have through bolt with backing plate on the inside and remember, there is that headliner that will probably have to be cut back to get a backing plate onto something solid (there is a void between the bottom of the deck and the headliner). Putting the ring under the mast step will be cleaner and much, much stronger. Also, if you make it with attachments all the way around, you can put on more turning blocks for whatever you haven't thought of yet.
Purely opinion and I've been wrong LOTS of times.
-randy