How do I do the following with iMovie?
For me, iMovies' user interface is largely inscrutable. Perhaps that's because it caters to a video editor's workflow - something that's largely foreign to me. Anyway, this won't be a detailed answer, but hopefully enough to inch you forward. 


The main thing I've noticed with markers is that they are like magnets: when the pointer is close by, it seems to "snap" to the marker. The only real use I've ever found for them is that they can be inserted during video playback (M). This is useful if you're looking for a place to cut a video, or make a splice to insert something. Here's another take on it.
Try using the "Precision Editor" (in the iMovie menu bar: Window-&gt;Show Precision Editor while a clip is selected in the editing panel. It may also help to "zoom in" (make the clip longer) using the slider in the upper right-hand corner of the editing area). Cf
Yes. The way I use this is as follows: 


Split the clip (iMovie-&gt;Modify-&gt;Split Clip) at your chosen point.
drag one of the Transitions from the palette to the edit area in between the segments of the split clip
adjust the length/time of the transition with a double-click on the transition in the edit area 



The picture shows the sequence, but you may have to view it full size to see the details.



Again - I'm no expert; you'll learn more, and quicker, by searching for help on the Internet. And keep looking! Some of the most useful features in iMovie are well-hidden: I think the "Ken Burns Effect" may be the best of these hidden features.