1)
XenForo is extremely easy to develop for and most importantly, it's stable for both designs and add-on.
I use to use IPB and found that every release broke things and required updates. 3.4.0, 3.4.1, 3.4.2, ect... It did not matter there was always something to fix when updating.
My site has 80 add-ons installed. I can easily upgrade XenForo without the need to update another add-on. And with 10 styles installed, I virtually almost never need to change or modify them to accommodate a release.
And while on the topic of development and stability, there is also the security aspect. XenForo in the history of the company (3 years) has had zero security concerns in relation to it's own code. The exception only being an outside flash uploader, that was promptly patched. IPB had to push out 7 security updates in just 1 year (link) and in my opinion, that is a little concerning.
The community here is very supportive and eager to help. The support here is wonderful. I've never known a development where people seem to jump over one another in order to lend a hand, give advise, tips, and solutions.
2)
There is actually more than a few people who have incorporated XenForo with WordPress. What you want done can indeed be done with some modification. XenForo is very easy to customize.
3)
There are several popular portals / cms add-ons for you to choose from. Customizing them to fit your needs is also possible.
