1) Are we losing our leadership? I mean...I guess so. I don't think we've had a great leader type President in a long time. And even the one's we've had are often not as great as their reputation. I'm not really looking for a great leader though.
2) Pro-whatever/anti-whatever? Every President since Reagan has started their term dove-ish on Russia. Remember Bush seeing Putin's soul through his eyes? Remember Obama's 'reset' with Russia? Everyone thinks Putin is someone they can work with...until Putin stabs them in the back by invading someone on their watch.
As to China, the great gamble that we all took was that we could lure China to be decent by showing them how awesome it is to NOT be starving to death under socialism by engaging and supporting their move to a sort of half-capitalism. And it worked for awhile. Under Deng Xiaoping China saw some degree of softening (not as much as we all hoped, but SOME) and the greatest rise from poverty in world history. But then Xi took over, purged all but his loyalists, and now China is just a straight up dictatorship. The Chinese communist party after Mao was always afraid of the single strongman leader. Yet Xi is that.
3) Trump and isolationism - Yeah, isolationism bad. But 'kill us leadership'? I think you're exagerrating Trump's impact. Not that I blame you, there's alot of that going around on both sides of the aisle.
4) What would Biden do? I suspect the era of Russian naivete is over. The D's have turned Russia into a bogeyman of epic proportions and so it's hard to imagine Biden being as naive about them as Obama and Bush and Trump were. But I do suspect he'll end the trade war against China unilaterally.
5) What would Sanders do? Hard to tell. Sanders barely knows what day of the week it is. He's clearly senile if you listen to him at all. Sanders may be the one person in this election even more unstable than Trump. And that's saying alot.