Momentum intuition, had me thinking last night

You are right in your 'maybe...'. There is another external force (besides that of the snowball) acting on the tree and so linear momentum of (snowball + tree) is not conserved.

 

So isn't the answer that m_snowballv_snowball ≈ (Mtree)v' ? or is = correct?

 

I don't understand how anyone would think that elastic or inelastic collision is something to do with the relative masses or speeds. It's entirely to do with the construction of the objects. A ball bearing hitting the steel plate on the front of an Express train is a pretty well 100% elastic collision. Two equal masses of rice pudding coming together are an inelastic collision. A (real) duck and a truck is inelastic. Two ball bearings colliding is elastic. Need I go further?

Also, it is pointless trying to find errors in the Momentum Conservation law. The law refers to the total system involved and always applies; you just have to take everything relevant into account if you want to apply the Law. When a 1kg duck hits a massive truck, which is freewheeling over the ground, it is only the masses of duck and truck that need to be taken into consideration for that collision. If the truck is exerting significant force on the Earth's surface (e.g. driving), the mass (or moment of inertia) of the Earth becomes, strictly speaking, part of the calculation. There is a tiny change in the Earth's rotation - as there was when the truck was initially accelerating - when anything happens to the truck. At the end of the process, the Earth will still be rotating at the same rate.
The fact is that in collisions, any change in the Earths motion will be insignificant. If there is an elastic collision then the small colliding object will have its velocity of approach reversed.

When I learned about 'Dynamics' we were taught about the Coefficient of Restitution. This was the ratio of speed of separation after to the speed of approach before collision. This can be used to calculate what will happen in partly elastic collisions (very common events). You could set up equations for Momentum Conservation and for velocities before and after and find out what would happen as a result of a collision. This Coefficient is not easy to measure and it will often depend upon the actual speeds and masses involved (most objects do not behave linearly) but it provided hours of 'fun' for us.