Very long primary in plastic

Abridged answer: primary them for 6 weeks, and if your gravity is where you want it, then rack to one of your PET bottles and age. Simply, you could have some off-flavors as a result of yeast autolysis. For higher-gravity beers, you want to let the yeast do their work, but if there is going to be 9 or higher ABV once fermentation is completed, that is a hostile environment for yeast, and they can break down and leak stuff from inside their cell wall into your beer.

Not an expert, but my general rule of thumb is: after the krausen has subsided, let the yeast clean up some of the other compounds (the higher the gravity, the longer), then if you want to purely 'condition' your beer, get it off the yeast cake and let it mature by its lonesome.

The Worst that could happen is you ruin the batch (undrinkable) and end up with stained containers that give a strange flavor to things you use them with.

That's probably unlikely. If you use the plastic bucket for 4-6 months with the mead, you may end up leaching some of the plastic chemicals into the mead, giving it a plasticy taste. If you're set on using plastic, I'd go for the PET bottles, as they are actually meant for longer term fermentation. They claim to be as good as glass, but I've only read of people using them for beer, which is in there for a significantly shorter period of time.

Personally, I only use a primary fermentation for my meads, and I do it in a glass carboy. I have 14 of them going right now. If cost is the issue, I'd suggest hitting up your buddies for some investment capital. A promise of half a dozen bottles of mead can really loosen the purse strings. =D