I feel you there! We send emails to multiple individuals in one company, and I typically want to know which one of those people caused the company to be an opportunity and which email.
Whenever I create a program in Marketo I always create a corresponding Campaign in SFDC and sync it. With that in place, when a lead becomes a potential client we convert it as an Oportunity. Any other lead that led to that company is also rolled up in the same opportunity. Once the opportunity is created, all the leads and the associated campaigns are all viewable in one screen (Opportunity). If you scroll down to "Campaign Influence," you can then assess the "Primary Campaign Source" and the Lead associated to that action. It will also state the campaign in the "Additional Information" section under "Primary Campaign Source."
The one caveat though is when I create campaigns, I make sure I check all the statuses i consider a conversion. The converted assignment is what SFDC uses to determine First Touch Attribution.