#blocking requirements

[magicjudge]

What would happen if I gave all my creatures menace and some how gave all my opponents' creatures "must block if able"? My gut says they never have to block because menace complicates things.

All of your opponent’s creatures would still have to block as long as they have at least two creatures that can block. They can use them to block different creatures (with at least two creatures each) or pile them all on one creature or anything in between.

[magicjudge]

If skin invasion is put on a creature, it's forced to attack. However, the release notes say it's controller is not forced to pay any costs. Is this true of all costs or just mana costs? If so how is this different from bonded construct from origins?

You’re asking about a few things here, so let’s back up and discuss a some concepts first:

Requirements:

Sometimes creatures are required to attack or block. These are called requirements. When you declare attackers or blockers, you have to fulfill as many requirements as possible.

Restrictions:

Some creatures can’t attack or block at all. Some creatures can’t attack or block unless certain conditions are met. These are called restrictions. When you declare attacks or blockers, you can’t violate any restrictons.

Costs to Attack or Block:

Costs to attack or block are a type of restriction, since they prevent the creature from attacking or blocking unless mana, life, etc. is paid. You’re never required to pay a cost to attack or block, even if you could.

Putting it all Together:

So, whenever you declare attackers or blockers, you have to obey as many requirements as possible while not violating any restrictions. This can have some odd results. For example:

Your only creatures are Bonded Construct and Runaway Carriage. Bonded Construct is enchanted with your opponent’s Skin Invasion, which says that it has to attack each combat if able. 

It’s now your turn, and you’re about to declare attackers. What are your options? Well, we have a requirement for Bonded Construct to attack, but also a restriction that it can’t attack alone. Let’s look at the possibilities here:

If you don’t attack with any creatures, then you’re not meeting the requirement, but you’re not violating the restriction.

If you attack with just Runaway Carriage, then you’re not meeting the requirement, but you’re not violating the restriction.

If you attack with just Bonded Construct, you’re meeting the requirement, but violating the restriction.

If you attack with both creatures, you’re meeting the requirement and not violating the restriction.

Since there’s an option that both meets the requirement and doesn’t violate the restriction, you have to go with that option since it maximizes the number of met requirements. This means that your Carriage (which you were saving for later) and your Construct both have to attack here.