I had to develop a way to paint with acrylics that would allow me to spend less time mixing paint. Back in the 80's I started premixing many of my colors in small squeeze bottles and I am still using some of them today. Also, I use plastic palettes and store them in plastic food containers (sometimes in the refrigerator) to keep the paint hydrated and usable for as long as possible.

Sure - as often as possible! I'm always trying to find better ways of putting down paint to create interesting textures. That's part of the fun.

I've had lots of opportunities for experimentation on the cover paintings I've done for the metal band SEPULTURA. For the cover to their album ARISE, I assembled textures from photos and integrated them with painted parts, then ran the pieces through a copier several times to get a gritty and contrasty feel to the image. I reassembled the elements into a collage and painted into it some more. For the last step, I had a sepia tone photoprint made of the collage, mounted it on a board, and painted the final image using the print as my underpainting. It was like having an adventure in my studio.

It depends. I prefer to work from my imagination, but there are times when I feel the need to supplement my mental image of a pose with more information.
When I use photos, I look at several to get the gist of the light effect or manner in which the folds of the fabric are moving and then extrapolate and make up my own figure based on what I've learned. This is quite a bit different from tracing and/or trying to copy a photo, which rarely works. Photos lie, but they do capture information about details. What you decide to do with them is part of what makes up your "style."
Painting from life can be part of the process, but I am more likely to work from a model in my studio for my personal work than for an illustration.