Jamie is correct.
How do you plan to track email replies? You cannot do this automatically.
From your triggers it seems that you are relying on someone to log an activity in SFDC. That would work fine, but is someone logging those replies?
But there is 3rd party software out there that allows you to do that. Siftrock/Drift Email e.g. https://www.drift.com/platform/email/
@Tracy_Boesken - I'm a huge fan of this tool and I can't give it enough praise. But after Siftrock got acquired by Drift, the key component that you're looking for is not that easy to find anymore on their website. I guess it's here now:
The Marketo integration is very solid, and we couldn't live without it.