What is the best way to infuse mint into a tomato sauce?
Mint likes oil. And it likes water. And it likes alcohol. Like most complex flavors, mint is complicated. 

The greener, vegetal notes are going to be from compounds like chlorophyll, and will be alcohol and very weakly water soluble. The astringent, sharper notes are going to from compounds like menthol, which are oil soluble. 

In general, the faster flavors, the ones that hit fast and fade faster, are water soluble, while the ones that linger are oil soluble. Mint flavor is sold both as an extract, in a base of water or alcohol, and as an essential oil, in a base of... oil. 

As for cooking, I would skip the tomatoes entirely, and do a mint and parsley pesto with walnuts and olive oil. Why? I like mint.