I will measure my halyard in the morning. I think 6mm will be strong enough – the issue is handling it without pain!
I think the idea of a light weight genoa is a good idea, particularly in the med.
There are two types of Atalanta masts (assuming yours is wooden) narrow and deep section. The narrow was fitted to early boats and is for 3/4 rig. However it was always considerd suitable to take masthead spinnaker etc in force 3 or less. The deep section usually on later boats and is the same for 3/4 and mast head rig. In fact the difference between the two is minor, and confined to the masthead fitting having an extra tang for the forestay. So with that type of mast mast head sails are no issue at all.
You aught to be able to handle your halyards at the foot of the mast while stood in the main hatchway, so the extra cleats etc should not be needed. Have you tried that?
PS hold fire on the T2L to see what Chris thinks.