Google sheets: How do you forecast numbers each month in a range that's non-linear?

Here's the google spreadsheet example anyone can edit:

https://docs.google.com/spreadsheets/d/1U1dxhtPQZOlach6xV_C_hw3vkwA_73lqUan2VXX5-4A/edit?usp=sharing

And I need it to work with blank/unknown cells or a formula for filling in cells based on the ones available.

This is the closest answer I've found so far, but I can't get the formula/code to work:

How to interpolate data in a range in Google Sheets