Your analysis of the project at hand is dead on! You're asking all the right questions and your attic air sealing, insulating and ventilation project is really going to make your home more comfortable this summer and save you a ton of money.
Let's start off with air sealing your attic. In addition, to any penetration in your attic, like the ceiling light boxes and recessed lights, you need to air seal the gap between the wall framing and the drywall that creates your ceilings. The 2X4s shrink over time leaving a gap where air can leak through. Great Stuff works great here.
Make sure to squirt some foam into the electrical and plumbing holes in the top plates.
Chimney chases and plumbing chases are some of the largest unsealed holes in your attic. They are frequently not air sealed because they look like they're supposed to be there. To air seal a chimney chase, you need to use sheet metal and high temperature caulk.
There's a lot more on air sealing an attic here..