How do you blend and separate responsibilities between interrelated marketing roles like Product Marketing, Growth Marketing, Sales Enablement, and Content Marketing?

It's challenging because there are so many overlaps, but I think it boils down to clear, consistent communication. And maybe breaking it down into how you work (or don't work) together on a project-by-project level. 

For example, on a product release, your committee will likely include people from each of these teams. When you set out the plan for that launch it's important to clearly define who own what. After enough releases, you all should be in lock step for how you work together on those particular types of projects. 

Over time, apply this approach to other projects types and soon you'll start to carve out everyone's niche. 

In one case at my last company, we also worked with the product team to create a document outlining all of the responsibilities we "owned" or "supported". The idea being that each responsibility needed an owner, but it doesn't mean another team could be involved in a supporting role. If anything, it helped facilitate the conversation around who owned what. 

I'd also check out my previous answer about creating your PMM Charter. 

The answer to this question is so contextual. What product(s) does your company have? Who are they for and how do you reach them? Who already is in place in the company and what skills do they possess? And then of course all of this changes as a company grows. My advice is to sit down and look at the goals for GTM. From there look at the strategy that is needed to deliver on these goals and the capabilities required to achieve the tactics that support the strategy. Through this you can define the functions needed and outline the best roles and responsibilities possible against the needs you have, the resources you have and can hire. Over communicate where there is possible friction from the gray areas and put in place a process/way to check in on the R&Rs established as things change (new products, high growth, new competition, etc). When things (inevitably) change you can simply repeat this process again.