Non-WP developer hired: which framework theme is the optimal?

Hi all

Feel free to skip to the questions below:)

A bit of background:
I work at an semi-governmental institute that is in the process of developing a new website. Our current site runs WP and was designed by experienced WP developers, but I did not have a lot of control over the development decisions or process. However, we’ve dithced our old developers (contractual stuff) and now I have full control. Our current developers are not well versed in WP but are willing to work for it. Initially they opted for their own licenced CMS, but I want to avoid the institute becoming tied to the current developers. From the institute’s size, web needs and composition, I’ve concluded that WP is a adequate solution – also, my collegues are accustomed to WP. From what I’ve gathered, the ideal situation would be to choose a cutting edge framework/parent theme that we can keep updating without ruining the child theme that our developers will make, but…

The actual questions:
To spare the developers from too much wear and tear, and make sure the institute ends up with a
– user-friendlig cutting edge (HMTL5?),
– flexible (future-oriented, various media/service integration),
– SEO– optimized theme that is
– maintained for the foreseeable future,
which framework should we choose? And is the “parent/child” or “framework” solution even a good idea in this case? Am I asking for too much and being a brat?

Plus, it doesn’t matter whether it’s premium (PageLines, Thesis, etc.) or free (Toolbox, Bones, etc.).

Feel free to ask any questions, and thanks for your help!