Ok I have some new code, but it aint working as intended:
http://pastebin.com/m5e541754
It returns 4 words and images aint displaying anymore...
I am thinking of another solution. i am beginning to think shorten the post text is plain impossible....
The new plan: is to add a new filter so only the stuff between the new tags <rm>textextxt</rm> is shown on the blog page. But how can I auto add ... before the </rm> and the read more link (so this should be done while filtering)? And how do I get bbpress normal posts to hide the <rm> tags when viewing a post and do nothing with them? Adding a filter <rm> to do just nothing wont work I guess since it will interfere with the blog <rm>?