Is there anywhere that I can set the default transition to apply when dragging a Title Clip into the timeline? And perhaps some default keyframe settings?

I find myself applying the same transition with the same keyframed opacity settings, over and over and over again, so I'm hoping there's a way to automate it a little.

I can understand your resent very well. The only I know of and that I use on a regular basis is copying and pasting and existing transition within the same project, as copy and paste works. Sometimes Kdenlive is picky when pasting, so you may need to click and somewhere in the timeline yet outside any clip or transition before you can paste.

PS: the title darkening issue should be fixed now in mlt, but it will take some time to reach the repos. The daily Kdenlive tar already contains the fix. At least for me this problem has been solved.

Returning to this, is there a way for a project to have a transparent background?

I load project files as clips for intro and outro's, but I've recently run into a use case where a logo is slid in from the bottom, and then slides out of the bottom a few seconds later. Putting this in it's own separate project file to preserve the keyframing, while simply applying an Affine or Composite transition should allow it to be used in the main project, except that loaded projects have black backgrounds by default.