You've got some confusion going on here.

> The truth is that there is no gray zone. Marijuana, until otherwise classified by the FDA, is illegal.

Marijuana is expressly listed in the CSA, so it needs to be reclassified by Congress, not by the FDA. In fact the FDA does not have the power to control something under the the CSA (ie, to criminalize it); that power was bizarrely given to the DEA, ostensibly to be used on an emergency basis. This is how MDMA was made illegal (and many other substances in the phenethylamine and tryptamine families).

Now, are there legal gray areas? Heck yeah there are. Some exist in the form of ongoing legal controversies. While it seems to be settled case law at this point that, even though a constitutional amendment was needed to prohibit alcohol, the same isn't true for cannabis, it's not totally unthinkable that the courts might start to accept carve-outs to this rule. Raich vs. Ashcroft wasn't a foregone conclusion when it was decided, for example. A similar case today might even go the other way, especially if the plaintiff were a state rather than an individual.

Then, there are large swaths of untested legal gray areas. For example, if the DEA tried to revoke a doctor's license again with different details (for recommending or perhaps prescribing cannabis or cannabis extracts), it's very hard to say what will happen (we have a tiny porthole of insight via Conant v. Walters, but many unanswered questions remain).

> Many states have decriminalized it, meaning that they have effectively nullified the federal regulations-- though not technically the legislation, as none [that I'm aware of] exists.

Hard to unpack this, but basically no part of this is true.

"Decriminalization" typically means that possession is not a crime, but instead is handled as a civil infraction. Several states decriminalized cannabis in the 1970s.

Legalization typically means that both possession (within certain limits) and retail sale (within certain limits) are wholly absent as a statutory offense to law. Often (but not always) this includes cultivation (within certain limits). This framework - again, called legalization, is the state of local laws in states covering about 25% of the US population.

I don't even know what you are trying to express by "not technically the legislation, as none [that I'm aware of] exists" - each state which has legalized cannabis has either passed legislation (which of course you can read online) or done so through a ballot initiative (the terms of which you can also read online). Plenty of legislation exists.

> Like immigration, the federal government chooses to enforce (under Trump) or ignore (under Obama) sections of federal legislation and regulation as they see fit.

First of all, let's not whitewash the Obama years: immigrants were brutalized, detained, and deported in huge numbers. Deportations are happening at about the same rate today; there has not been a sudden spike. Some particular populations, especially refugees seeking asylum, are treated even worse now than they were then. But the government taken a very uncivilized approach to the border since the 90s.

I'm not sure how this relates to your points about cannabis, though. Perhaps you are referring to the Obama-era directive in the justice department that no federal resources will be used to prosecute people who are complying with state medical marijuana laws?

> I'd personally feel much more comfortable if everyone was subject to the same laws, as selective enforcement can quickly lead to corruption where one group is punished, while the other is ignored.

Well, yeah. :-)

It's not just a matter of comfort; equal protection under the law is part of the bedrock of the Western legal tradition. The 14th amendment didn't invent or establish it; in fact, some of staunchest of the radical republicans insisted that congress had the authority to enforce equal protection even before its ratification.

Drug prohibition is a clear affront to equal protection; each stage of it was enacted specifically as a pretext to criminalize particular populations, and never as a public health measure.