Best rope for main halyard

Heaven help us all.

For what it's worth, I'll probably use 4mm Excel Vectran on the 3k when the current Selden Kevlar runs out. Presently it's doing very nicely having an inch or so cut off the top before each Nationals!

How can I put this?

Farc anal, imagine you are an angel holding up the sail by its head. You are supporting only the loads in the sail, which as you say are induced by kicker, mainsheet & cunningham. This is the halyard lock situation. Mast locks do reduce compression (as well as stretch as you say)

Now, instead of holding the sail-head, you're holding a pulley which has a main halyard running from the sail head and back down to the ground. You will have to pull twice as hard as before as the sail now has a 2:1 purchase on you.

Reduce the halyard tension by putting a block on the sail head and dead-ending the halyard end at your hand, and the overall load on you will go down as there will be less total loading in the elements connecting you and the ground (sail + halyard).

I call a halyard 2:1 if I have to pull the halyard twice as fast as the sail goes up the mast. This is what most people mean, but it is a matter of definition, as you demonstrate.

perhaps you are miss- interperating my simplified crappy sketch above .

put more less   simply , compression is product of down force generated on the mast tip , through combination of downhaul/cunningham tension , halyard tension , kicker,mainsheet , rigging , gravity ,  all acting together . to create a compression force .

once rig is set up doesn't matter how many purchase you use - you are only trying to create the SAME compression on the mast for set conditions , the loading on each individual purchase is less as you add purchase , but together add up to the same sought after compression .

off course you can increase compression by adding more purchase , but as  above you are only looking for a specific amount of compression at any time, as you say yourself adding 2:1 halves the load on halyard - actually most halyards are two to one think you mean three to one if you want to nit pick .

The max compression is when tack /cunnigham is pulled down as far as it will go with head as high as it can be .

Mast locks work not because they reduce compression- they don't  , but because they eliminate halyard stretch , no less load on mast at all . they can be a pain to use at times , but they  work ,thats why they are used in top competion where possible/allowed  , VOR , Americas cup , Finns , Etchells , humble cats ,Ok's etc etc

PS My Bad , figures are wrong above , the example of the 3:1 halyard on right should show 33.33333333333 kg pull on each leg giving the SAME 100 kgs Compression , in all cases compression generated by factors expressed above