1) Thank you Jacob for strictly enforcing safety rules. That's the #1 most important thing in my book.
2) As you can see, everyone has their own opinions and practices regarding weapon construction. Really, the only way to be safe is to look at weapons in person and get experience with them.
And the only way to be sure that your weapons are safe is to constantly inspect them for safety. A weapon can get damaged at any time. You need to check it often, several times during an event. In LE, I'd recommend checking each of your weapons every time you reset.
3) Regarding thrusting: Well-built weapons can be safely used for thrusting. The important part isn't the open-cell foam squishy tip. That tends to break down over time, so with a used weapon, it provides little protection. Also, being open-cell foam, a core can rip through it fairly easily.
The important part of building a thrust-safe weapon is the closed-cell foam plug. The outer 5/8" pipe foam insulation should extend 2" past the tip of the core. A 2" piece of closed-cell foam should be inserted as a plug into that gap and taped securely in place. Underneath that piece, the tip of the core needs to be covered so the plug can't get pushed down into the pipe. Lastly, a disk of closed-cell foam should be firmly taped over the tip of the whole thing (plug and 5/8" foam) so the plug / core can't push out past the 5/8" covering foam.
4) As for how fast a weapon breaks down, that depends on how much you use it. I know players who attend fight practice 3-4 days a week and event 3-4 weekends a month. Obviously their stuff breaks down faster than someone who hits 6 weekend events a year.
5) 5/8" foam can be hard to find. I recommend ordering it from DoItBest hardware
They will ship free to a local hardware store in their network (which is huge, there will be one close to you), or can ship to you direct.