Not quite the same set up as mine
On my '79 O'Day 23, the forward reefing line is attached to the mast on the port side as you describe. It then leads up through the first reef point cringle, then up to the second reef cringle. From there, I lead the line down to a turning block mounted near the forward end of the boom on the starboard side, and secure it to a cleat, also on the starboard side of the boom about two feet or so aft of the block.At the aft end of the boom, the reefing line is again attached to the boom on the port side, lead up to the sole cringle, and then down to another turning block on the starboard side of the boom (near the end), then lead forward and secured to the same cleat as the forward reefing line.To reef, I loosen the halyard and haul down on the forward reefing line until the second cringle is as close to the gooseneck as possible, securing the line back on the boom cleat. I then haul in on the aft reefing line to shorten the aft portion of the mainsail to the only cringle, and secure to the cleat. If I am doing this operation at the dock before leaving, I make it nice and neat and tie off the reef points. The only time I ever had to reef out on the water, I didn't bother; with jiffy reefing systems, reef points are cosmetic. I think you are right when you observe that the system of two cringles near the mast but only one at the foot of the boom means that the original foot of the sail will only be parallel with the boom if you take in one reef. If you use the deeper reef, the triangle of the main essentially pivots on the aft cringle. I have never used the first reef, going right to the second. However, I assume that the sailmaker knows more about this stuff than I and that the first reef has utility. Maybe I'll give it a shot next time out. By the way, I still use the original sails that came with the boat (in surprisingly good shape, but due for replacment). I can't quite picture how your forward reefing line is trapping the sail slides. I keep my boom fairly high; when the main is fully raised, the gooseneck is only a few inches below the cutout in the mast. This allows for plenty of headroom in the cockpit and also allows for the use of the slide stopper pin to keep the slides from coming out of their track when raising or lowering the sail.