Thanks for your note.  I'll accept being practical, but I'm sorry to say
I'm no expert in the "arena of glue".  My formal education was in
business administration, and I worked in the chemical industry for 30
years, but as an executive with limited knowledge and understanding of
the technical side of the business.  My brother and I ran our own
company manufacturing industrial adhesives, sealants, potting compounds,
encapsulants, mold-making materials, and point-of-use mix-meter-
dispensing equipment for multi-component reactive materials.  Our
products were based on epoxies, polyurethanes, natural and synthetic
rubbers, silicones, polyesters, acrylics, but no hide glues!

I can try to answer only a couple of your questions based on life
experience and a few educated guesses...

> 2.  What's the effect of the water content?

It probably guarantees shrinkage, and in the case of a hide glue,
it must also retard the setup time.

> 3.  Is there a better "thinner" than water?

Yes -- at least in the context of the materials I am familiar with.
We used to adjust the viscosity of our epoxy compounds by adding
non-volatile or low-volatile dilutents that would take part in the
chemical reaction, rather than evaporate.  Because the technical
adhesives we made were ratio sensitive, "thinning" by the end-user
would be discouraged since it would change the formula of the compound,
and therefore change the resulting chemical, electrical, and physical
characteristics of the cured material.

For example, mixed epoxies can usually tolerate the addition of a little
petroleum solvent to lower the viscosity for a particular use, but is only
moderately successful, and then only if the pot life of the material is
very long so as to permit the solvent to flash off before the ultimate
cure of the epoxy.  One of the benefits of using an epoxy is that they
can be 100% solids -- no solvents -- and therefore they don't shrink.

I wish I could be of more specific help to you.

Best wishes,

Jack Hardman