I think you need to ask them more than just if they are moving or know someone who might be moving....like are there any FSBOs on their block or in their neighborhood....any vacant houses....etc. 

Going over elements of the numbers will be expected by investors to some extent, but I would suggest not to hand out anything that looks like a representation of the success of your deal.IMO, the key to a good pro forma is keeping close to actual historical data, assume that expenses will increase and income will remain flat, or with proper justification a slight increase.

There is one for the city I believe and maybe for other justifications.

Justifications for rent increase should also be included in your rent increase letters.

I have a better justification of baking in the repair cost if the I'm collecting closer to market rent..I'll increase the rent and see what happens.

I can't see any numeric justification to do otherwise.

Throwing out offers without a justification is a reflection on you.You can bet that if a listing agent has to present some unjustified low ball offer they will do so to that owner with comments like "I can't believe this agent would even allow this to be made, I suggest we not even counter this".

And what's your justification for buying another in this high market?

They can use their pre-payment as justification to stay. 

I'd think too that if the association had some other purpose, like promoting something in common among all the resident members it would certainly add justification for the existence of the association.