Once a lead has been synced with SFDC and is assigned an owner/queue, if assigned that owner/queue directly through Change Owner, can you later call up the sync lead "use auto assignment rules" for the first time on this lead, or is this not possible once the lead has an owner at all, whatever the source of that lead owner assignment came from.
Yes, you can still use the sync to SFDC use assignment rules even if someone is already assigned to that lead/contact. It's just syncing that lead/contact and it won't change the owner or go through the lead assignments rules unless your SFDC team has it for a particular reason to resend them into the lead assignment rules. Essentially, Marketo is just throwing the lead/contact over and SFDC needs to do what it is programmed to do with it. Usually the lead assignment rules in SFDC only add people in it if the lead doesn't have an owner or queue.
There are some limitations, though: the SFDC LAR will only fire on lead creation.
Vote here: https://nation.marketo.com/ideas/7524 and look into the idea to get a workaround.
Answered a similar question yesterday here: https://nation.marketo.com/thread/50658-can-sync-to-sfdc-use-auto-assignment-rules-be-called-more-th...
Check if this helps.
No, currently, the LAR will only fire once, on lead creation and will not fire again when the lead is updated nor when the lead owner is changed.
Vote here: https://nation.marketo.com/ideas/7524