Use marketing as a means to gather more data and test. Marketing is one of the most underrated and undervalued skills in startups. Startups don't struggle with competition but getting attention. Marketing drives more data into the product, thus you can validate your theories. Pre-product market fit marketing should drive acquisition and customer development.

Groupon made a billion dollar business out of email marketing. I don't think it is underrated by people who are good at growth. Most startups do think email is boring and rather invest in third-party platforms. This is a mistake as email is the true social network and reflects the raw habits and beliefs of users. It is hard to do email well because there is so much noise and crap users see. I open notification emails all of the time if they are valuable to me (ergo. answer this question). Just use the golden rule: would I want to open an email like this?

Depends on the life cycle of the product. At the most meta level, you have to have retention to have a product; however, the biggest driver of growth will usually be acquisition. If you don't retention, you don't have real users thus you don't have a user base.