Disregarding the utility and or convenience of Android apps on a FxOS, the problem is exactly the “parallel part” of your idea.
Think in the lower level. You have 2 engines fighting for the processor, that’s not how software works, to solve that, then, we would need another layer to manage these 2 to work well, so we are already increasing a layer, which would make the phone slower even if you are not running any android based apps.
Also, you need to think about the integration with the JS APIs and it’s utility as a technology in general. The idea about making technology better and more accessible is not about making you able to run Instagram and twitter, it’s about making a better environment and affordable phones, that’s why FxOS is focusing in emergent markets, where android phones are usually expensive besides the shitty ones.
Mozilla thought the web had all the ability to make this, but it was still short on certain APIs, so they are making new ones for the sake of being universal, so for example, you have a battery API that works not only on phones, but also on laptops, how amazing is that?
Then, you must consider: Is it interesting to make an API to communicate with Android Apps? Will it help evolve the web? Will it open new possibilities? For the user, yes, sure, they will be able to install Instagram, though as @gerv said, you will end up with just a bad android phone.
Now, will it evolve the web? I doubt it, Android programmers will just continue making android applications and giving no support for other plataforms, and when it stops working, it will not be the creator, but the platform who will take the blame. Do you think Mozilla should be blamed if Instagram stops working? Not only that, would developers want to do that? Blackberry tried to force a bunch of converted android apps on the blackberry store and they got sued because they had no permission, BB did all the “hard” job, but most developers just didn’t want it anyway.
Where would a normal user get an APK if he can’t enter the Play Store because he doesn’t have an Android phone, would developers release their apks on a secondary story? Amazon got their own store, and they only met success thanks to the kindle fire (which does not have/can’t enter the Play Store), so developers reached them, so unless you make some cheat or illegal stuff, there would be no way to get these APKs legally.
Of course, a solution to that is a dual boot phone, you boot either Dalvik or Gonk, but not both at the same time.