Just to make it more interesting, let’s say the plan was to hijack the plane, not crash it. (It does not make sense that it would be put on a slow descent if the intention was to take it down.) Then let’s assume the outside pilot is the bad guy, who already incapacitated the inside pilot and set the plane on a slow descent, left the cabin........could he get back in?
It makes perfect sense if there’s a mountain in your way and you want to fly into it, praying to Allah, undisturbed by screaming passengers. Why dive steeply when the land will rise to meet you?
Yes, he could get back in. The outside pilot can always get back in, unless the inside pilot intentionally locks him out. It’s not like walking out the house door without your keys.