1.Are you trying to install a bathroom in the attic? The soil stack must extend uninterrupted thru the roof. If you install a toilet at the top of the stack with out any bend you will omit your entire venting system. The stack must terminate above the roof. What you are proposing is to cut the soil stack before it exits the roof and installing a toilet is prohibited.
2. A lav and tub cannot be tied in like that. That wye application only works for toilets.
3. Depends on your local code. In Chicago you need 4" thru the roof.
Can you post pics of what your trying to do.