Step Sequencer/Session Drummer - multiple loops in same song

No, no need to load another instance of SD (or any other synth). The MIDI routing is handled by the MIDI track's in and out, not by the step sequencer (or the MIDI "part" of an instrument track if you're using an instrument track rather than separate MIDI and audio tracks).

Well, OK, you can alter channels and all sorts of stuff within the step sequencer, but one thing at a time :-)

Really the step sequencer is just an alternative way of programming MIDI to the piano roll view or the staff view. Each method has strengths and weaknesses depending on what you're doing and what working methods you find suit you. 

The step sequencer it's excellent at handling the kind of stuff that "old fashioned" hardware sequencers are/were used for, drums being the "classic" example, or synth lines that sound "automated", if you see what I mean. 

One useful trick is that you can convert step sequencer clips to MIDI clips and back again if you need to.

One caution - if you're using time signatures that aren't "something/4" it can be a bit quirky/buggy. Such time sigs generally aren't straightforward in a hardware step sequencer either though, so I suppose it's being "true to form" in a way.

A final tip -  if you've not already done this, go into preferences and set the MIDI buffer to longer than the default 250ms (e.g. to 500). This makes the step sequencer far happier, especially if you're inputting while it's running (it can kind of fall over its own feet sometimes). If it does start glitching, stop Sonar and reset the MIDI engine (one of the icons under the clock in the control bar).

The way I do it is simply to copy the verse and paste it in after the chorus, repeat as necessary.

The matrix view is probably useable for this kind of thing as well, but it's not something I've used much (it was very limited in its abilities prior to X2) so can't really comment.