theres no such thing as typical plastic.
The reason water has an expiration date is because the cheap plastic used in retail water breaks down relatively fast and dumps toxins into the water, making it unhealthy.
You need to specify exactly which kind of plastic you are using in order to know how long water will last in it.
Fun Fact: Most emergency water is kept in aluminum packets because it doesnt break down quickly.
Yes, there absolutely is a typical type of plastic used for a lot of different products. Pretty sure they have to specify the type it is now too so you should be able to find that out. For the water bottles the OP is talking about there are several plastics that would be expected (although they have gotten more variety, especially after people started freaking out about certain types).
That is bullshit. Average plastic water bottle (as in if you buy a bottle of water) is PET, and that doesn't leech (let alone break down in any meaningful way) unless put in heat (and even then they have versions that are heat resistant) for extended periods of time.
Yes, you do need to know the type to know how it will fare, but you are grossly exaggerating this. Not saying it isn't a problem at all, and yes some are absolutely worse (BPA for instance), but it's not at the levels you make it sound like, and typically requires pretty excessive heat or certain conditions.
The aluminum pouches are for various reasons, but they are for emergency use and so need to hold up to a wide variety of environments. I'm guessing they probably have other benefits (i.e. I'd imagine they'd hold up to heat, so you could use them to gather water from the environment and boil it in the pouch to kill bacteria).