Hate to break up all the rankings but Joe Russo did a Q&A and if nothing else clears up what they were thinking in regards to Cap's adventures in time.
Q: Did Captain America's action at the end affect the timeline? Does that mean there was a time where two CA existed in a same universe?
A: To me, CA's action in the end wasn't the fact he wanted to change anything, it's more like me has made a choice. He chose to go back to past and lived with the one he loved for the rest of his life. The time travel in this movie created an alternate reality. He lived a completely different life in that world. We don't know how exactly his life turned out, but I'd like to believe he still helped many others when they were needed in that world. Yes, there were two CA in that reality, it's just like what Hulk said, what happened in the past has already happened. If you go back to past, you simply created a new reality. The characters in this movie created new timeline when they went back to the past, but it had no effect to the prime universe. What happened in the past 22 movies was still canon.
Q: EG's plot, is it a parallel universe or a closed time loop?
A: Nope, not a time loop. Both Ancient One and Hulk were right. You can't change the future by simply going back to past. But it's possible to create a different alternate future. It's not butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam.
Here's the link for anyone that wants to see for themselves.
I find this part particularly interesting
If Cap were to go back into the past and live there, he would create a branched reality, Joe explained. The question then becomes, how is he back in this reality to give the shield away?
The brothers smile.
Interesting question, right? Joe said. Maybe theres a story there. Theres a lot of layers built into this movie and we spent three years thinking through it, so its fun to talk about it and hopefully fill in holes for people so they understand what were thinking.
ok if they've thought about old cap and there is some explanation, fine. ---
...Snrk. And theeen, we do stuff like, 'Persona!' with our Personas and beat the crap outta Shadows...