The original analysis is correct- the compression on the mast doubles if you support your mainsail by using a halyard that runs up the mast and is then secured at the bottom. Instead of thinking of the halyard and luff of the main independently, think of them as one continuous line. Negating the friction of the sheave at the mast head, the tension in the line would be equal throughout the length of the line. So, since the line travels up the mast and back down, there are essentially two falls. Pull on the downhaul with 400lb and that equates to 800lb of force on the axle of the mast head sheeve compressing the mast.

It is the same as having a block and tackle with a 2:1 purchase.

sm