the problem with bottle caps ?

1) Everywhere it states that bottles with a cap only can be stored vertical, nowhere is explained why this is and what happens with the bottle cap otherwise?
* Does the protective plastic degrade (faster) and thus exposing the beer to the metal?
- - - YES &NO. It may degrade faster, beer is acidic, but it shouldn't do so in a few years. Getting on to a decade, perhaps yes, but certainly not to the point of exposed metal. I would think contact with chemicals from the plastic would be a bigger concern.
*IF the inside has oxygen scavenging posibility, will it release the enclosed oxygen?
- - - NO, but it might not scavenge it. I believe it has to be upright for that, not in liquid.
*Cap can pop of the bottle because laying the bottle down creates to much pressure?
- - - NO, no more so than if upright. That shouldn't happen with a properly packaged beer. Low filled bottles are something to avoid, they'll put more pressure on the exposed areas.
*..... ? - - - Some will argue that laying a beer on its side exposes more surface area to the oxygen already in the beer but I don't believe that to be the case.

2) How is it possible that oxygen can enter the bottle?
*the plastic has a very small leak?
- - - That's always a possibility, but probably not. However, you never really know. Caps are generally spun around in a "pool" of other caps before they find their way into the cap chute and they could become damaged. Another issue is that a cap may not be crimped on as tightly as it should be which can happen for a number of reasons.
*with time the plastic degrades and so letting some oxygen in and pressure out?
- - - It will degrade, dry out, and shrink but only over a very long period of time, longer than you should be aging for sure.
*it can travel trough the plastic? (should not be the case)
- - - NO, oxygen cannot fully permeate the plastic until it breaks down after many, many years. It would go around it before that happened. There could be some minor transfer though, but I doubt that would be any worse on the final product than the oxygen already in the beer.
*....?