If you had 99 str and you were bored pking without it then sure but otherwise no because looking at the combat calc you are 56cb without it and with it you would be 60 combat but if you could get 93 str for that and have the same cb
Max hit 80str 31 pray rune scim :196
Max hit 80str 31 pray rune 2h : 227
Max hit 93str 1 pray rune scim: 196
Max hit 93str 1 pray rune 2h: 239 +12 max hit
the difference is larger in lower levels but remember this is unpotted with only minimal str bonus
Prayer points don't last long enough to be worth it imo, also recharging is a pain in the ass and protect item doesn't save you much in F2P. I'd personally keep prayer as low as possible for F2P Pking.
Only worth it if you PK with a Gravite 2h which you obviously want to protect, and 31 pray makes it hit so damn harder. Also if you become good in prayer flashing, your strength boost can last quite long.
No, each prayer level is 1/8 of a combat but you can only level combat on even prayer levels. For example, 51 prayer will always give you the same combat level as 50 prayer even if you were, say, only .075 combats from leveling, but 52 prayer will be 1/4 of a combat higher than 50 prayer.
Honestly 43 is only useful if you pk p2p or if you're maxed and just want the extra prayer points. Since he's kind of low level I would strongly advise against 43 - it's 5.25 combats that just disadvantage you at lower levels