We know Ron left Revelstoke and was last seen in that area about 1430, 3 miles west of town. Based on that fact the winds aloft and the performance of the older C172 I have the following information.
With a cruise speed of 110kts and a climb speed of only 75kts and a winds aloft speed of 30-40kts depending on altitude the C172 was flying, I have come up with the following.
If you leave Revelstoke at 1430 and have an average ground speed of ~70kts(due to head winds), and fly direct to your destination on a course of 228deg, in 1 hr 43min you will be over the exact spot and time the US Border Patrol Aircraft spotted a target, at ~1615.
Now there could have been 2 planes in the same place at the same time (We call that a mid-air in Aviation) but that too would have been seen on radar, but what are the chances of 2 planes headed that same direction at the same time without meeting in the air ?
If you continue on this same course with the same average ground speed you will arrive at Qualicum Beach in 3hrs 42mins .
This would result in a landing time of 18:12 or 4 minutes past sunset with a total trip mileage of 256nm well within the range (515nm) of the older C172
Little bit of information on pilots...when they have some place to go/get to they use the most direct route and save the flight seeing for another day after all planes are not cheap to fly.
If you figure in the price of fuel, oil, insurance, hangar and routine maintenance it would cost about $110 (USD) to fly this C172.
Here is a image of the path I have calculated...notice how the straight the line is, even over the point where the radar spotting took place south of Spence's Bridge.
I have been doing these type of calculations in Aviation for 39 years (long before GPS) and teaching them for almost 24...I think I have a handle on this stuff.