Disclaimer, I'm not a mathematician, physicist, or even of notable intelligence... but I too have seen a lot of movies :)
I agree it's unlikely to be possible that we could send a person or object back to another time or forward in an movie trope kind of way...
What if everything exists at once, but there's an unalterable sequence of events. So, like you say everything has already happened past, present, and future including any kind of time travel.
But, let's say time travel is possible, but is limited to small state changes in the building blocks of the sequence of record (dare I say, the blockchain of time).
In theory we could make these changes and they would be completely undetected until we understand both how to create the change and know when and where to look for the changes.
Now suppose in the future we find a way to do this and collectively agree on some kind of location to start a linked list of information from other "times".
Thus begins some kind of technological singularity. Instantly we gain access to all the knowledge anyone at any time in existence that has the means wants to share.
Now our problem is how to process that much information in a way to make it useful and trustworthy in a reasonable amount of time.
This isn't well thought out and I'm sure there's some kind of paradox in there somewhere, but it was a fun Friday musing. I think this kind of allows for "time travel" to be possible via encoding information instead of killing Hitler while also allowing for those changes to have already happened without altering our currently observable past.
If anyone knows of any sci-fi stories or movies that explore a similar concept I'd be totally interested in the recommendation.