Each fixture needs its own vent connection - that doesn't mean you can't combine them according to the rules so you only need one penetration (minimum of 42" above the floor or 6" above the flood plane of the fixtures, whichever is higher). There's a limit on how far you can run the trap arm before it is vented - this depends on the diameter of the drain line - smaller=shorter distance allowed.
If you're not planning to use both the tub and shower at the same time, if it was my house, I'd consider putting in something like a 6 or 7' tub/shower combo (tubs generally come in various lengths at 6" increments from 54"). This is big enough to actually soak in rather than a 5' tub that has maybe a 4' well in it. this would also allow you to keep the storage shelves, which is nice to have in the bathroom.
1/2" pex split for the shower and tub would be a poor situation if both were used at the same time. Also, if you have more than one showerhead, it might start to be an issue. The tub will fill slower than if you had 1/2" (or larger) copper pipes. My 6' tub takes about 8-9 minutes to fill with 1/2" pex.