Google Analytics is a powerful tool, as you say, and as a result it has a steep learning curve. With that said, calculating the ROI on a marketing effort is not as difficult as you make it seem.
Using goals and ecommerce tracking, you can see the (dollar) return from any source, cohort, or time period. All that's missing is the input (investment), which only you know. Divide your investment by the return and you have your ROI for any segment you choose.
The entire issue with last-click attribution is not really an issue. Not only can you change this setting, but also you can view "Multi-channel funnels" and see every touchpoint (channel) of a user, even if they didn't convert on the first visit.
From a quick glance at Tend, it looks like your value offering is showing the same multi-channel funnel that GA shows, just with a nicer interface. That's fine, a better skin can be a sufficient value prop, but it's dishonest to frame this as though you're providing information that GA does not.