[SalesForce] Populating a suppression list in Marketing Cloud From Sales Cloud

Journey Builder sends cannot be currently captured in SalesCloud because Journey builder cannot include Salesforce data extensions as their event source (even if SFDC ID is used as the Subscriber Key). The product team at Marketing Cloud, I believe, is working on facilitating this in the next release with Data Stream features.

You can certainly order rows by date and select the oldest using a partition. You could also join the results to another data extension to compare dates. Something like this:

select
  y.emailaddress
, y.subscriberkey
, y.insertDate
from (
  select
    x.emailaddress
  , x.subscriberkey
  , x.insertDate
  from (
    select
      d.emailaddress
    , d.subscriberkey
    , d.insertDate
    , row_number() over (partition by d.subscriberkey order by d.insertDate asc) ranking
    from DataExtension d
  ) x
  where x.ranking = 1
) y
inner join DataExtension2 d2 on (d2.subscriberkey = y.subscriberkey)
where y.insertDate > d2.insertDate

I'd also like to throw out the possibility of using a Server-Side JavaScript Script Activity in your Automation to delete the subscriber from the list.

You can retrieve rows in a Data Extension and iterate through them. Then for each subscriber to remove, retrieve their Lists and remove the unsub list from the array. From there you can just Update the Subscriber object with the new list object. I've done this quite a bit in custom preference centers that manage publication list membership. The only difference here is that you're iterating through a data extension to get the subscribers.

If Script Activities aren't enabled in your account, SFMC Support can enable them for you.