>would God find it a problem to do more charitable work to animals than humans? By that I mean volunteering at an animal shelter instead of a homeless shelter or giving out food at a food bank
The first one is good. The second one is better. You know what's worse? Not doing either. Be careful with stop doing something good just because you could theoretically do something better. Confront evil where you find it. Helping animals is better than not helping anything. On the topic of animals and souls, I have no idea. They're God's creation, and unlike us, who ate from the Tree of good and evil, they act in accordance with their own nature, in accordance with God's will. Maybe they're saved by default, if in a different way than how humans are saved?
Regarding the second question, I think you're getting the wrong idea about Heaven. You're thinking about it all in a way too physical manner. Try to think of Heaven more as a "state of the soul" and God as a principle of being, an energy, rather than a being (since He transcends so much our common understanding of what a being is). And, if you actually get to Heaven, you can save others through sincere mutual love. To those who love a lot, a low will be forgiven.
It also seems that your conception of God is too similar to that of the philosophers. The Christian God is not a demiurge; He came to Earth to die, and die in the cross, to save us all by His suffering and humiliation. Ours is not a God who arbitrarily condemns us to suffer but one who's with us through that suffering.
Try to live out your faith and develop a personal relationship will God (usually done through daily prayer). It'll help you understand a lot more things about religion. Religion is not something you just understand logically, is something you live out, something holistic that involves much more than just the intellect. Look up to those who you find holier than you and imitate them. That will help you deal with your conundrums.