Is it possible to start and use Ardour (with mixing and recording) without jack but pulseaudio?
No.
The next major release of Ardour will allow you to use the program directly with an ALSA device, without JACK, but it is unlikely that you will get satisfactory results if you point it at the PulseAudio pseudo-device.
PulseAudio is a desktop sound server and is not intended to serve the needs of music creation and pro-audio software. There are several ways to create a bridge between PulseAudio and JACK so that they can share audio, but in general most of the applications you might want in a music creation/pro-audio workflow can already use JACK. PulseAudio does not have any facilities within it to share audio between applications - to do that you have to get the audio “out” of Pulse and into something else (e.g. JACK).
Ah, now with some better background information I understand the problem. But this makes it hard to jam in Ardour to - for example - a youtube video.
We are not responsible for the overall state of Linux audio. If you use a Linux distribution intended for music creation/pro-audio workflows such as AVLinux or KXStudio, you won’t have any problems doing this, because they ensure that JACK and PortAudio interoperate correctly.
I have had success using the ALSA JACK PCM plugin to route audio from non-jack applications, including Adobe Flash (youtube). Details here:
For youtube, i.e. flash specifically you can use libflashsupport-jack, see http://repo.or.cz/w/libflashsupport-jack.git
A lot of people will find that youtube serves them HTML5 video now, and even more in the case of vimeo. Flash solutions are less and less important (but still handy).
If AVLinux or KXStudio would support 64 Bit I’d think about to change my distro from my beloved Arch. Even if I have to mess around with ppa’s and git’s.
I’ll give a try to libflashsupport-jack. Thanks. 
Ah, great. I’ll first gibe it a try in my VM. Maybe its the right thing for me?