I have a song comprising free-recorded audio-only tracks, no midi, no adherence to measure/beats.
 
The guitarist fluffed the first chorus so I need to copy/paste all tracks of the second chorus to the first chorus.
 
I put start/end markers around the bad and good chorus using the bass drum track as a guide.
 
Now I want to zoom out and copy/paste, but I cannot get the marked area to snap to the markers.
I have gone to the options and selected Landmarks > Markers, and then set Snap intensity to Extreme.
I have tried smart, non-smart grid and snap-to-grid, snap-by-value, by Measure, Ticks, Samples, Frames, Seconds ... every combination. Nothing works.
 
Whenever I try select the source chorus in the time bar, then zoom in, the marked area did not snap-to-markers.
 
But if I select, say, snap to whole measures, the cursor snaps to measures.
How does Snap to Markers work?

It also seems to be impossible to zoom in, start the selection from the first marker, then zoom out to complete the selection to the second marker because you cannot extend a selection. You can only start all over again.

There are lots of ways to do this. The one you're using should work, but make sure to disable Musical Time Snap by clicking the button, so that only Landmark snap is enabled.
 
Some other ways are:
 
- Click in the timeline between markers to select that region.
- Use From=Now and Thru-Now to set the selection with Now time at each Marker (my keybindings are non-default, so I'm not sure of the defaults)
- Split the clips at Markers and lasso select them.
 

Select all
Open the Markers View
Click & drag the markers you want your selection to be constrained to

I solved it in Audacity(!)
 
But I gave it another try with Sonar and discovered the following:
 
The secret is to move the mouse cursor into the UPPER HALF of the timeline. then I get a magnifying glass and an arrow pointing down. If I now click between two markers, the area between the markers is selected!
 
I can now cut and delete hole to remove the bad chorus, but copying the source chorus is not working as expected. I select the area as before and Copy.
 
I then insert at the point where I cut previously with Paste Special with Slide over old to make room. And what is inserted is not the selected area I copied between the markers, but instead from the last marker to the end of the song. I tried several times now. Same thing each time.
 
Anyway, as I say, I did it with Audacity and my work is complete.
 
Thanks for the help!

Instead of deleting the hole, and pasting with Slide Over To Make Room, leave the hole, and paste the new chorus into the hole in 'Blend' mode and tweak the gap/crossfade as needed (I'm assuming both choruses were roughly the same length).
 
Both Delete Hole and Slide Over To Make Room can be troublesome, especially the latter which is one of the few 'known issues' that is consistently mentioned in the Readme. The Bakers have recently committed to improving the arranging tools. In the mean time, I recommend managing 'holes' manually.

Thanks, noted this for next time.
 

brundleflyThe Bakers have recently committed to improving the arranging tools.

Oh, I really hope so. The other two DAWs I use are better, but not by much.