MarketPress Theme compatibility

Hi -

We're using MarketPress (along with WPML/WPML MarketPress) on a new bilingual travel ecommerce site, as we've found it to be a good, solid and straightforward platform for online sales.

However, we're experiencing some problems, with errors being generated on some of the core MarketPress pages ("Call to a member function options() on a non-object in /home/OURSITE/public_html/wp-content/themes/impression/page.php"), and associated "Add to Cart" buttons not appearing on MarketPress pages.

I contacted the Theme developers (AIT Themes), and was told:
"Problem is that plugin wants to find some settings of theme, which don't exist in this case. Plugin wants to use template page.php of theme, but plugin should use its own template. We have not used template page.php for example. To make it work, theme coding need to be changed. " Seems like AIT have thrown a lot of WP conventions out the window when coding their themes.

We want to avoid coding a new Theme from scratch, and so my question to WPMUDev is:

What are the requirements of a Theme (particularly page.php) to get MarketPress to successfully integrate? Realistically, is this something that our developer can fix within a day, or do we need to go back to square one with a new Theme?

Any help appreciated!