A few (too many, probably) comments here:
Paddling up the narrow sections between those lakes is pretty much a non-starter.
For the short channel between McManus and Smith, just take the portage. If the water is low, the takeout can be a bit of a pain due to rocks; at moderate or high water levels, no problem.
The longer channel from Smith to Whitson - depends on the water level. At high water, the approach from Smith features fast current and you have to work even to get to the takeout. The portage itself is a good trail and not difficult. As MartinG says, the current at the upstream put-in can be pretty lively, but if you're reasonably strong paddlers it shouldn't be a problem. However, at very high water levels the put-in can be drowned and you'll have to get wet to get up to where the current is a bit less aggressive. That said, I don't think the water level this year will be extremely high. At present it is somewhat high, but that will probably ease up over the next month.
If the water level is low, it can be difficult to get up to the takeout since the water is shallow and it's difficult to get a paddle in deep enough for a good push to fight the fast current. It is possible to line/wade up the channel from Smith to Whitson on the right side at low flow. There is one deep hole partway up that will mean getting wet, and launching at the top can be tricky, though at least it's shallow. Wading up on the left side is fine until you get to the top, where current and rocks make it difficult to get back into the canoe.
Both channels are certainly runnable on the way back down. Whitson to Smith, the current weaves back and forth in the channel, so you need to follow the main flow. The river bottom there is mostly gravel, but there are nuisance rocks in a few places. At low flow it's easy to fetch up on a gravel bar, but not a big problem. Smith to McManus is also fairly easily runnable, especially at higher flow, but at low flow there is the same possibility of grounding on a gravel bar.
Anyway, don't be put off by all these details. If in doubt, just suck it up and do the portages. Whitson is a nice, scenic lake and you will probably have a pleasant time there. Best wishes for your trip.
