The most likely idea for how Tortsky ends up head guy is that Lenin lives longer and Trotsky is his right hand man for long enough that his "heir apparent" status outweighs his political weakness.
As mentioned, Trotsky didn't want to spread revolution willy-nilly. Indeed, part of Bolshevik ideology was that revolution can only spread to countries with the right level of political and economic development - which, when you think about it, can be used to give you plenty of reasons to NOT intervene in your neighbours and have friendly relationships with whatever republican democracies, monarchies or dictatorships might rule them.
Trotsky WAS a believer in trade and openness. Stalin's impulse to scientifically isolate the USSR REALLY hurt Soviet science in the 30s. Without that happening, Soviet scientists continue traveling abroad to conferences, sharing their results, seeing the experiments of German, British and French scientists, continue getting international prizes for their work. Foreign scientists continue to be able to travel to the USSR to work, to have conferences etc. More open trade means both a more dependent USSR but also a USSR that modernizes faster, that has better quality goods overall and a generally more efficient economy.
Trotsky would also be weaker politically. This means the USSR never leaves the collective leadership system as it did under Stalin and (somewhat) under Khrushchev. The country would be ruled by committee. That means things would be less focused - both good and bad. No persecution of millions just because one man feels under the weather one day.
On the other hand, the tyranny of middle-managers is rightly feared across the globe.
Almost certainly the Soviets develop a space program - the Bolsheviks were major tech fetishists. If the "Trotskyite" USSR avoids WW2 then it is quite possible that they'd have the level of development to start a serious space program in the 40s.
It would be fun to see a friendly space race between such a USSR and a Weimar Germany as the two used rockets and space exploration to reclaim the prestige each country had lost after WW1.
fasquardon