There is less halyard stretch, although most dinghy sail designs have a softer luff than the stretch in a typical modern line used as a halyard so I expect it is imperceptible in a dinghy of this size with a dacron sail.
The mast compression is the same whether you locate the halyard cleat at the bottom of the mast, half way up the mast or at the top of the mast. As one naval architect explained to us......."Hold up a broomstick in your left hand with a 40lb weight tied to a line going through a sheeve at the top of the broom stick.....if you tie the line off by the sheeve or at the bottom of the broomstick, the compression is still 40lbs.......compression is affected by the tension on the main and the equal and opposing force provided by the mast, not where you tie off the halyard"
Another trick to prove this......apply max luff tension with the halyard and the mast lock off.....so the mast is pre-bent under compression. Lock the mast lock so the halyard is locked... release the halyard. The mast will not spring back up to vertical under less compression.
Old wives/Etchell sailors lore.
The definition of mast compression decides whether you think the compression changes. I agree that halyard cleat location does not effect compression at the bast of the mast - that is provided by mainsail forces, standing rigging (not applicable here) etc. However, the mast will be compressed between the head of the sail and the location of the cleat by the halyard tension and any cunningham tension applied to the sail. Having a halyard lock, or cleat at the top, ensures that this compression is only experienced by the very top of the mast, which is the definition being applied here.
With an Aero, the halyard cleat is external to the mast, so with the amount that the rig bends it must cleat at the top. Otherwise, the distance between mast tip and where a mast base cleat would be would shorten too much as the rig bends, reducing halyard tension and allowing the sail to sag. If the halyard tension is kept high enough to ensure a tight luff when the rig bends with a mast base cleat, you would, as Amati states, simply create a bow.
The definition of mast compression is the down force being applied through the walls of the mast which is equal and opposing to the up force exerted through the halyard (lifting against the sail) and shrouds (lifting against the deck).
When you tighten your shrouds, you will create a similar "bow" effect , even though the shrouds are effectively "locked" at the top of the mast. Next time you are going out on your Aero, apply aggressive halyard tension with the halyard lock off ...sufficient to apply prebend to the rig.....then lock the halyard...et voila, the prebend is unchanged.
Now agreed that if you stand at transom and pull halyard, you are creating an additional force which is not compression , which will bend the mast.
There are a variety of forces that the controls of a sail boat can apply to the mast. Some create compression. Some do not. Its all fun to figure out.
Force can turn a corner. The outhaul on a boom is similar. The compression along the boom is the same whether you cleat at aft of boom or at front of boom.
Not quite... Due to the sheave at the top, the force the sail pulls downward with is doubled on the top of the rig. With the cleat at the very top of the rig, as on an Aero, this doubling in force is only acting on the top 1 or 2 inches of rig. If the halyard was cleated at the bottom of the rig, as in many other dinghies, this 2:1 action acts over almost the entirety of the rig - from the sheave to the cleat point. Any bit of rig below the cleat point is unaffected by this, and only affected by shroud tension, rig forces etc. This is the reason big boats spend thousands on 2:1 halyards (halving compression between the top of the rig and the cleat point) and halyard locks (eliminating halyard-caused compression forces). This enables them to use lighter rigs, lighter halyards, less hull structure...