For me, him surviving at all is pushing it (not to mention totally undermining his incredible exit). But if he's going to survive, spending a year bush camping in the Upside Down is the worst possible option. Him being the guy in the russian cell is the second worst, unless the Russian military has teleporters or some weird connection between their systems created a second portal in Russia for just a second and he was blown straight through the Upside Down to Russia (which would still be terrible). The best possible option is probably that someone (Russian or unscrupulous Fed) dragged him out unnoticed and held him prisoner somewhere in the US. The upside down is only an option for me if you reveal something totally game changing. Like, say, the upside down really is the future or has totally screwed up time in different areas, so that he finds a way back after a week or so but only arrives months later. Or maybe the Mind Flayer has a use for him and a way to keep him in some sort of suspended animation.