Installing Box Newel Posts on pre-fab stairs
We are building a new house and have installed pre-fabricated stairs. We are ready to prep for the box newel installation before the wood flooring/stairs are finished and have hit a snag. We have been advised to use a "box newel mounting kit" to install 6-1/4" newel posts on the starter step and 5-1/2" newel posts on the landing. Using this system would keep us from having to cut a square into the step/landing area.
When we looked at this system in detail, glue, pin nails and one lag bolt make up the strength and we aren't confident of sustained strength over time with daily use. Are these systems strong enough? If not, what is the best way to install a hollow centered box newels? We can't find any solid wood newel posts and were advised that they will warp/twist anyway since lumber isn't kiln dried like it used to be. We feel the newel post should be set down into the step for strength, but we don't know if our thinking is correct. We have access to the sub-floor from the crawl space on the start step, but not on the landing.
Any advice you can offer will be helpful. Thank you.
Note: The starter step shown on the attached photo link of our stairs was made too narrow. The pre-fab company is coming to correct the width so we can mount the newel posts off to the side as shown in the second photo link.