BobF
While the structure wasn't what I would've done starting from scratch, it did perform flawlessly and the structure made sense.  

+1, it was "weird" at first but then it "clicked" in how it made so much more sense given that a track is anything you want it to be in Reaper.

Anderton

Also note that exporting tracks as whole stems (so from the project beginning) is important for 100% perfect way. While Sonar is sample accurate, REAPER is (by default) time accurate. 

Can you explain why that matters? Sonar exports tracks starting at measure 1, which is the same as time = 0 in Sonar, so it seems Sonar tracks should align perfectly in Reaper.

In case stems are exported from the project begin and put to the new project begin, there can be no problems.
 
But in case someone export let say from measure 3 and then insert at measure 3 in another DAW, samples/measure can be not aligned. Depending from different settings in originating and target DAW, (a) WAV samples can be not aligned with project samples (b) the clip/item position can be not exactly measure 3.
 
That is hardly audible nor visible. But can produce unexpected results in artificial tests (f.e. if you try to null with another track by phase inversion) and/or editing operations (f.e. cutting at some measure boundary and repeating, in case the beginning of the clip/item is not perfectly measure aligned, resulting cut clip is also not exact number of measures in length, after repeating 10+ times that can produce real audible "shift").
 
Related questions periodically pops in the REAPER forum. Each flexibility has its price... REAPER does not have fixed project sample rate, allows audio content with arbitrary rate inside and allows to have Measures/Beats/Items at precise time position (which is almost always  not on the project sample boundary). As the consequence, samples from the audio source can be somewhere "between" project samples. What happens then depends from the operation, source/project rate and many settings. Sometimes "current ongoing source sample" is used, sometimes "nearest source sample" is used, under some conditions REAPER does interpolation, from simple up to "full flavor" sample rate conversion with LPF. For 99.9% users in 99.9% projects all that is unnoticed. But once in a while, people observe side effects