I’ve seen claims that Iron Maiden was the first “full stage show” east of the wall, and also claims that Hellion was the first US band to play there at all. Who was first?
Those two statements aren't mutually exclusive, since Iron Maiden are NWOBHM. Maiden indeed toured the Warsaw Pact and Yugoslavia in 1984, as part of the "World Slavery" tour. The tour opened with 5 shows in Poland, and also featured one in Hungary and two in Yugoslavia: one in Belgrade and one in Ljubljana. This leg of the tour was documented in a short film released at the time which the band called Behind the Iron Curtain. Certainly at the time, the media made note of the novelty of the tour: Billboard's Ethlie Vare, writing in the October 1984 issue, wrote about Iron Maiden "penetrating the Iron Curtain" as the first full metal stage show to do so.
Hellion's tour was in 1990, just after the fall of the USSR