My friend climbed Unicorn, Boundary and Steven's a couple of years ago. I've kept an email route description that he sent me. Perhaps some of the following will be helpful...
...Boundary is easy. From the summit of Unicorn, proceed down the ridge, moving slightly (50 feet or so) to the right when necessary. There is a substantial cliff on the left for the last 100 m or so of the ridge before you get to the peak. Ascend Boundary via a third class rock groove on the west / southwest side of the peak. Time from summit of Unicorn to some of Boundary: maybe 20 to 30 minutes.
From Boundary to Stevens is a little more tricky. What we did was head South from the Boundary summit until we got cliffed out on the ridge. Then we cut down a steep vegetation gully to the right (West) about 300 vertical feet, and then traversed left (South) through a lot more vegetation until we got to be an easier part of the ridge to the south of Boundary Peak. This does trample on a lot of the alpine flora, and I don't feel this is the optimal way to go, but it worked well for us.
I think the best bet is Beckey's suggestion. From the summit of Unicorn, go south along the ridge about 100 m. Start heading down left (East) off of the ridge. When you drop about 200 feet, you should come to a bit more of a rock drop off. Move south along the edge of this rock band and look for a lower angle spot that you can easily get through. This should be third class with no rapelling required.
Once you get through this upper rock band, began traversing to the south, trying to maintain a fairly constant elevation. In earlier to middle season this should be on moderate snow slopes and a bit of scree. This should lead you to the large pass area between Boundary and Stevens peak.
From the pass, start heading up towards the Boundary summit. Stay to the right initially and you will avoid a small cliff band. On the lower slopes of boundary is a large area of really lousy rock. You can fourth class up through it, but there is big-time rock fall potential, and it would be sort of tricky to down climb, especially in a group of more than two people. The route we took to get through this was to head straight up to the ridge line to a low point. The skips the entire fourth class action and puts you on the ridge with easy walking to the summit. Time from the pass to the summit, approximately 1 hour.
To return to the Snow Lake trailhead, retrace your steps to the broad pass area. Head north down into a beautiful long snow gully - great glissading! Descend about 600 feet, and look to your left for a short two to 300 foot ascent that leads to a low pass. Ascend to the pass. At the top of the pass, contour west for about 500 metes, trying to maintain a constant elevation. This should bring you to the base of the large buttress that comes off the Unicorn summit block, and to the top of a ridge that overlooks Snow Lake. Head north down this ridge about a quarter-mile.
Look down into your left, and you should see a large forest of slide alder. This is the downhill epic bushwhack that gets you back to Snow Lake.
Warning: do not try to head straight down to snow lake before you get to the slide alder patch, or you will get cliffed out!
Descend approximately 600 vertical feet through the slide alder, playing Tarzan is you go, until you get to the camping area on the north end of Snow Lake. Pick up the main trail from the west side of the lake and return about 1 mile to the cars.
