I assume these house rules are for RtL and not for the traditional aspect of the game.

Second can't be implemented without modification of monster card.

I think house ruling that way can cripple some huge and massive monsters. It seems to be common that huge and massive monsters get clogged in halls, even by shrinking and expanding. Now, if they can't move shrinking and moving with one square, I think they could get stuck easier.

Firstly, I don't think that Descent has make great job to balance game for different number of players. Sometimes, adding new monster would be pretty insignificant (one goblin archer and fourth hero) sometimes it is pretty huge.

So first question, why they choose to use old scaling system ?

Second : how I can homerule four activations with any number of heroes and which amount of health will be enough to compensate missing heroes ?

Mmm...I think it is a big deal for the traditional aspect of the game, while having another goblin represents two more actions...here in RtL is...well, one more attack.

Although, as you say, it sometimes seems to don't affect at all. But I think what you said applies to certain monsters from the base game. I mean, it will not be too much difference to add another Zombie or Goblin Archer, but it will truly change if there is 1 more reanimate or 1 master Kobold.

The same thing applies to duo (mostly large) monsters, there is a difference between having 1 minion, or 1 master or having both.

For example, remember that in SW:IA figures can move through figures, while in Descent you cannot, so having 2 large (mostly massive) monsters can hurt any hero composition, even with the extra activation, in the way that they can get trapped easily. Like imagine having two shadow dragons, they will get to you and it will take too much time to kill them both (or Ice Wyrms, even worse if you get KO).

Another thing, I think 2 heroes will get swarmed if you use all monsters, perhaps 3 will have more chances, but 2...I mean 2 figures against 12 kobolds or 2 golems...I think they won't make it, not because they won't have the power to kill the monsters, but because monsters can possibly get stronger.

Nonetheless, If you still want to house rule this, I consider that 2 to 4 HP would be nice, 2HP for a 3 hero-game and 4 for a 2 hero game. I mean, if FFG considered 2 HP more wasn't big deal when they add the Tainted mechanic, I assume that amount of HP would be ok. For the activation system, well, for a 3 hero game you can use the same rule from SW:IA (heroes can decide who among the 3 will have an extra activation, and for a 2 game, both will have an extra activation). For the tokens, you can use the objetive tokens.