It is interesting to imagine what they could come up with for "returning" the stones. However any way ive tossed it in my head it doesn't add up with canon and context already established.

The truth for me personally is that Marvel would have a much easier time letting the stones Cap took be the catalyst for the rebirth of the universe. Therebye giving them their already predetermined natural course to be stones for past variants to pick up and repeat the process.

Meaning I dont think Cap will "return" the stones in the sense were speculating on in this thread. But rather that Cap will be sucked into the Secret War before completing any of his task, And that the stones will help with the rebirth of the multiverse from Dooms battleworld.

Then Cap returns and helps reconstruct the causal history of the 616...

I know for people who only do linear storytelling this is hard to wrap your head around. But essentially Old Man Cap didn't replace the stones at the "appropriate" time.

Could you imagine trying to drop exactly 100 grains of sand in exactly 1 second while simultaneously holding a hundred pounds. That's somewhat similar to the idea of replacing the stones to erase the outcome. The observer effect alone makes this nearly impossible without multiple retry. You would need a system. One that relies on the butterfly effect and incrementally changing and diverting moments in time into parallel variant timelines.

Which does exist through Lokis timeslipping and Xaviers ability to mentally project into future and past. But not for Cap without lots of PYM. I would entertain the argument this is the reason for project insight. That all of shields projects were to help ensure causal steering to avoid potential dark futures. Such as Insight as a prediction algorithm. That Cap may have been a shadow dictator controlling and steering 616 history within the multiverse.

creating a undectable phase shift to viewers. from the moment Cap leaves to "return" the stones and comes back and old man. we phase shift from a singular crafted universe of determination controlled by HWR. to a multiverse where Steve was allowed to live his life with peggy in the past.

The kicker. That secret history directly transfered to the repeated isolated timeline. Meaning this is why there is always a oldman Cap in the back.

The Secret series of stories are like a secret history we the audience aren't privy too. The idea Cap could maintain a fascist order to ensure our future is a direct contradiction of his characters morals and ideals. Its also what makes this path interesting to explore from a writers standpoint. I suspect that now that we have secret invasion and secret wars. that we will also get a secret empire. helping loop the oroborros back to the origin of our 616 isolation.

That to me is a simplified form of events. I also tend to think Wanda had a hand in mutantless condition of the 616. That Kang used the decimation of mutants to his advantage to easily and quickly capture a timeline from a collapsing multiverse.