Basically, yes; IPC serves the same basic functionality as WP, but don't expect it to be as powerful for blogging or as easy to use as WP. You do have the option to stick with WP when you switch to IPB, but you won't be able to achieve the same level of integration that you would be able to with IPC, of course. It's really up to you whether you value the functionality of Wordpress over the integration of IP.Content.
The IPS homepage is actually built on IP.Content. ;)
I had a similar problem with the board not sending the verification emails, and it wasn't in demo mode. Check the email address you're using, and take a look at the SMTP settings and see if anything looks off. That might be where your problem is originating from. You'll find those settings at /admin/index.php?adsess=xxxxxxxxxxxxxxxxxxxxxxxxx&app=core&&module=settings§ion=settings&&do=setting_view&conf_group=10. It turned out that I had these settings configured incorrectly.
That's a good point, but I just don't see how they be actively developing for both 3.4 and 4.0 at the same time. Nothing will get done for either if they aren't putting the majority of their focus towards either one. And ATM, 4.0 is surely taking priority.
That being said, a 3.4.7 update might be something they could consider right after the 4.0 release, if there's enough things that need fixing. (I personally haven't had many problems with 3.4, but I'm a relatively new customer.) The problem I see with that though, is that they'll be needing to fix early bugs with 4.0, so that would stand in the way of a possible 3.4.7 update.
I'd really like to see 4.0 released as quickly as possible, because I'm developing a site that's going to be totally integrated with 4.0, and I'd love to get a beta and be able to start developing it soon. ;)
It's unreasonable to say that IPS is totally abandoning 3.4 though. It's not at the end of it's support life, and it won't be for quite some time. No, they're not going to waste their time releasing another 3.4.x update when they have 4.0 to worry about, but they'll continue to release security updates and provide support for clients still on 3.4. What more do you want from them?
Are you looking for single sign on? If so, there are a few options.
Wordpress IPSConnect. This seems like the easiest way to go. http://community.invisionpower.com/files/file/6089-wordpress-ipsconnect/
There's IPBWI for Wordpress, and although I've never used it before, it's supposedly more difficult to get working. http://straightvisions.com/community/store/category/1-ipbwi-products/
Then there's also Invision's very own SSO development services. This seems like the ideal way to go if you have some cash to spend on it.
I'm looking into adding SSO to some of my Wordpress sites as well, but I haven't tried anything as of yet.