"random":0.01,
"dimension":0,
"mob": ["minecraft:skeleton"],
"result":"allow",

"armorhelmet":[{"item": "minecraft:skull","damage":"0"}],
"armorchest":["minecraft:chainmail_chestplate"],
"armorlegs":["minecraft:arrow"],
"armorboots":["minecraft:arrow"],
"helditem":[{"item":"minecraft:stone_sword","Count":"1"}],

"healthmultiply": 0.3,
"damagemultiply": 1,
"speedmultiply": 1.25,

"potion": "minecraft:invisibility,10000,1"
The resulting skeleton does not hold the stone sword properly please help me

If possible please answer me another question blaze cannot spawn on lava
{
"dimension":0,
"maxheight":20,
“ block”:“minecraft：lava”，
"mobs":[{
"mob":["minecraft:blaze"],
"weight": 1,
"groupcountmin": 1,
"groupcountmax": 1
}]}

As of 1.12.2 skeletons will not hold a stated item because they are already given a bow

I believe the issue with the blazes on lava is there may be a space in the ID where there shouldn't:
minecraft: lava = no
minecraft:lava = yes

The brackets you are using look strange, so if they are wearing the armor provided (they should) then you are fine, but if they aren't check these examples:
(older version)
(newer version)
If it still seems to not work check your JSon here

note- I am not the mod creator of either mod, so I'm not sure what version you are using

using "onjoin": true fixes this!