Eventually you'll find other sources - any waxy plant can be used in fractional distillation, for example. It's not so much expanding production, as it is sustained farming techniques. That is, even if it reduces production for a year, switching to harvesting partial sections and adding some sort of fertilizer. (Probably animal waste for the first couple of years, then probably just nitrogen fertilizers. They're not that hard to make) Plif will be the most important because 1) It's next door, 2) they already use the wax, so that means the reeds that are destroyed to extract the wax are available, 3) cellulose plastics are easiest to make with the equipment they have available. They'll be listed in any of the reference books in that genre, because they are so basic. (Heck, my _animation_ history book had information on the production of nitrocellulose plastics used in the film and animation industry)

Increasing production can be done in wetlands, you just have to identify the areas you want to go into. Growing up around the Gulf Coast, there are LOTS of areas that people can go into, but won't, because it's too mucky even for the hunters. Plif is almost certainly like that. They'll just have to figure out how to reach those areas, and how to maintain and harvest them. It doesn't require damage to the wetlands themselves, or trying to artificially expand it. Just don't harvest all of a section, then wander off and wait for it to grow back naturally. Harvest _part_ of a section, then move and harvest part of the next. Just like forestry. (Grasses you can do a bit more to, of course)

Coal tar? Yes, you can use fractional distillation to extract paraffins from coal tar. (paraffin wax)

http://pubs.acs.org/doi/abs/10.1021/ef400470v

The interesting thing about distillation and what we think of as the petrochemical industry is that you can make shorter chain organic molecules from almost any long chain organic molecule, as long as you provide the necessary heat and pressure. You can even make longer chain molecules from short ones, but it's a more expensive process. The benefit here is that it doesn't matter that it's a more expensive process - it's the only game in town. The biggest upside to the added costs? Incentive to recycle and reuse rather than trash. Any plastic can be reprocessed if you want to do it. (Thermal Depolymerization is the most efficient form of this. You can toss anything organic in it, and get out sterile water that can be treated or used for non-potable uses, light oil, minerals, and natural gas. It's smelly, but so is a tannery.)

So, in this case, I'm looking for the next 10 years - the kick starter of industry. If they get the basic distillation process down, they'll be able to have high energy/low mass fuel for balloons, or maybe blimps. That'll get them high enough to start making mapping plans to explore beyond the valley walls - and even flash signals across the valley over longer distances. With _enough_ fuel, they can put engines on a zepplin and use it for trading over short distances beyond the valley walls. None of that requires too much more than they already have. (No, hydrogen is NOT that dangerous in a blimp. Even the Hindenberg just burned slowly, from the top down. It didn't explode. Everyone ran away from it. We abandoned the technology far too early. I'd love to see what they could do nowadays, with the better engines and fabrics we have)

Eventually, they'll find either plants like the jojoba, that have a high wax/oil content, and can be grown easily in a commercial setting, or a source of petrochemicals. Either way, they'll be a lot more focused on reusing the resources by then.