Product analytics tools like Amplitude and Mixpanel get very expensive at scale. How do companies replace them?

Sure, it's correlative for technical reasons, iTunes / AppStore does not provide (yet, might change) a mechanism for that, so 3rd parties have to build their own. I recently discussed with people from Adjust and I was really impressed. HasOffers / MobileAppTracking seems to be a good choice as well.

NO, there no current tools available.

Try Intercom.io You get visitor level information on how active they are etc.

Much, if not all, of what you are looking for can be accomplished in Google Analytics using their Conversion Goals. I am not sure about *all* of it - specifically the information you want the merchants to have access to. There are tools that do analytics better and more detailed for applications like this, but GA is the best for your price point. You will want to plan for growth and moving to a more comprehensive solution, as your web app grows. Questions, comments, concerns, gripes, compliments or complaints? Book a call.

I have been managing multiple domains/sites for my clients since a few years now and I think the easiest and best way to do it in your case would be setting up simple WordPress sites on each one of them and managing them with something like ManageWP(https://managewp.com/) . If they're niche domains, you can monetize them through ads or use them as affiliate sites. Selling them can also be also an option. If you can get the sites to look fairly nice and if they are niche domains, you can sell them on sites like Flippa(https://flippa.com/) . Overall, you have many options. I'd be glad to help you in detail through a call if you want.