The first time I was on a trip that included the South River the plan was to return to the Tim Lake access point from Craig Lake via the South River on the last day (5'th day) of the trip.   We were extremely lucky to run into two park rangers on North Tea Lake.  They informed us that the portages on that route had not been cleared in 9 years.   There wasn't a hope in hell that we could do that route in 1 day but fortunately the rangers were able to contact two of our wives in Huntsville and they shuttled our vehicle up to Kawayamog so we could get home.  I was able to do the South River a year or two later after we heard that the portages had been cleared.   Even though both of us were in good shape, traveling light and able to move quickly over the portages it still took us two days to get from the Tim River access to Craig Lake.  We did that trip in May when water levels were good -- I'm sure it would be slower going in August.

The 3640m portage runs along an old railroad embankment that used to carry a logging railway from the town of South River.   It's a nice smooth, level portage except that as the map indicates there are no bridges/culverts where the rail line crossed the river.   I would expect you would be able to wade those crossings in August but with the volume of water we had in May they were quite tricky for us.

You likely would not see anyone else on the South River part of your trip.

Low maintenance portages and campsites are signed just as regular portages and campsites are.  Obviously, if a sign goes missing for some reason it will take longer to get replaced.   The condition of low maintenance portages is highly variable.  A portage through a mature hardwood forest could stay in good shape for many years without any maintenance.   A portage through a dying spruce forest could have a large number of deadfalls even if it had been recently maintained.  Overall I would say my experience with low maintenance portages has been good.