Any information on monster HP scaling from single player to multiplayer?

1 player = 100% monster hp
2 players = ???% monster hp
3 players = ???% monster hp
4 players = ???% monster hp

is there going to be a PVP arena added to the game at some time?

Is there any way to give friends items?

"Whilst [multiplayer is] completely optional" This is such a lie. Some monsters, like the Behemoth and Ancient Leshen, are designed for a team of four and do not scale up when multiple people join the hunt. Meaning they have the same amount of HP if you're alone. The "Multiplayer scaling" section also makes it pretty clear that the game is designed to make it easier if you're bringing people along. HP x2.2 VS DPS x4. I love the entire series to bits, but honestly, as someone with very few friends, this makes the game so much less appealing.

Any info on the new 2-player scaling?

These comments are wrong. Read article here from IGN and Capcom. Basically there is no scale for 1 player, 2 players gives a bit more hp and difficulty, and 3-4 players are max difficulty and HP. 3 players are at a disadvantage a bit since you lose your palicos and still have to deal the same damage as a 4 person team and take hits just as hard. If all 3 players are not too terrible it can be dome though.

"If the amount of players drops back down to 1 in a hunt that was multiplayer at one time, the monster HP will not reduce back to normal." this is no longer a thing. the monster will now scale down if you lose players in a quest Proof:

Don't read any thing posted below or above this comment. Every things posted is sad and confusing. The game is easy as ***** solo. Each player added will increase the hardness. You just may not notice it since you get player spamming attacks. But you will notice most of the monsters solo can and will take far less time to hunt then in mulit player. Same for the big hunts made with more players in mind. Only a few hunts will be truly hard til you better understand the game and find their weakness. Just work on finding ways to damage and evade tank fights. Game is player knowledge based. No exp levels, just gear and knowledge. So don't be lazy and just learn what works from playing. Good Luck.

Here's how it works (*you can check it yourself, by using an overlay):
SINGLE-PLAYER: 100% monster HP
TWO-PLAYER co-op 150% monster HP
THREE / FOUR-PLAYER co-op: 220~260% monster HP (*depending on monster)

Notes:
• The HP does NOT drop back when players leave a co-op -- making players leaving / lagging out early in a three-plus player team, very annoying for the sap left alone (best to quit and restart versus monsters like Behemoth and Kulve)
• The Palicoes DO return after players leave / lag out
• Two-player co-op is effectively the easiest, because it's only 50% more HP in toto (25% more for each player), and two Palicoes -- i.e., effectively a four-player set-up, especially if the Palicoes have decoys, because the monsters cannot aggro a single player / stop players from continuously dealing damage