I'm preparing to build a railing across the back of a terraced patio area in my yard to prevent people from accidentally stepping off the back edge. It's a 3 foot drop so a railing is necessary.

I'm building the railing using 4x4 posts with 2x4 top and bottom rails with round aluminum balusters in between. All wood is pressure treated.

Questions:

1) My posts will be flush with the top of the rail, and then I'll be adding a 2x6 cap. What is the best way to attach this cap? My first thought was to use #8 2 1/2 inch stainless screws from the top side. I've seen people suggest that drilling down through the top will allow water to seep in and cause the wood to decay faster. So then I started thinking about possibly attaching it from the underside instead, but I'm not crazy about toe-"nailing" with screws because I fear splitting the 2x4s and/or having the screw points poke out through the top. I also don't have a countersink bit, though I could pick one up. When screwing from below, 2 1/2 inch screws are probably too long, so I assume I'd need something shorter.

2) I was planning on cutting all my posts to final size before installation, but I saw someone online suggest that you should leave the posts longer than needed and cut them after installation by snapping a chalk line with a level. How do people normally do this?