I watch over Trinity project for as long as 3 years. I want to give my grade on its development activity. I think that the project is not capable of public use and rather suited for small group of inside hackers.
Along the history of development there were too much lame unprofessional decisions. It was fulfilling some strange unpractical ideas. In fact, it came to a logical unsuccessful end.
While in hindsight some of the project's goals were a bit too ambitious, this has been true at some point for many open-source projects over the years. A better measure of success is whether or not the project is still alive and moving forward, along with an active and engaged userbase. At this time I think we can answer both questions with "Yes!".
If you install 3.5.13 you will get too much trouble. I think, that implementing all the missing features into KDE4 is less cost, than waiting when this woe project will get to a decent condition.
I encourage you to hack on the KDE SC 5 sources to fix the issues you have experienced, and then work on getting the resultant patches into KDE SC 5 so that all may benefit from them. AFAIK the KDE 4 series has transitioned into maintenance mode at this time, so it is highly unlikely that you will get anything UI-changing accepted into the KDE 4 series branches.
Just my $0.02. :-)
Tim