Hello... I know you don't specifically say that K2 can work with this template, but if I install K2, is it easily adapted? AND, most importantly, will it be a good responsive page? If I have k2 categories in 3 columns on the page, with it collapse down into a single column at full width in the mobile phone view?

Thank you for letting me know.

All our themes are working with K2 but the current K2 version 2.6.9 is not responsive.

Are native Joomla 3 articles fully responsive? Is there any reason to use K2 any more?

Yes Joomla articles are native responsive.

Problem solved.