Over-building can be overkill in L.P.R.
Why is a steel line in a replaceable mount? You might be better off slipping the steel end loop under the upper bend of the engine hook. A steel leader will never have to be checked or replaced for that matter - it won't degrade or melt.
The purpose of the replaceable Kevlar mount is to check the line after 10 flights or so and replace it if needed. You can't replace a Kevlar line that is tied inside, under the forward centering ring.
In his photo example, the top of the steel line extended a few inches beyond the top end of the body tube. That's a zipper waiting to happen.
To see how a replaceable Kevlar line works, check out the article I wrote in the Apogee Peak Of Flight. https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
As far as I know, I came up with the idea back in 2013. I'd hate to see the technique used incorrectly.
Steel leaders have been used for years, Centuri pre-installed them in the engine mounts of the old Mini-Max line of kits. At the time, a Mini-Max F was considered high power. Currently, North Coast Rocketry uses a steel leader in their MPR kits.
Forum posts have shown some people fiber glassing LPR body tubes and fins. There is no reason for this, unless you are practicing for a much larger, faster model.
"Papering" is one way to strengthen fins. Just don't add too much weight to the back end of your model, it might end up unstable.
Years back when a C engine was the limit everybody was more concerned with weight and a higher altitude. Now weight isn't as critical, most people fly for sport.
Estes kits now use a very stiff wood for their fins. Papering isn't really necessary for strength. Many builders paper their fins rather than fill the wood grain.
Don't misunderstand - I build my LPR models strong enough to last many flights, but I don't over build. I use epoxy when needed, I paper fins on occasion. But, that's about it.