I am using acetone to dilute the gelcoat, will it cause any problems at the final product ?

Do not use cleaning solvent or Acetone thinning (diluting) resins or gelcoats as this can lead to added pinholes, surface problems and premature yellowing. If you need to thin the gelcoat you can use Poliya GTM (gelcoat thinner mix) you might as well use styrene up to 2-3%. If you use more you will degrade the resin or gelcoat.

Why, Polipigment color paste is cured - hardened before the expiration date ?

The main reason is you most likely stirred the color paste with a styrene, resin or gel coat contaminated stick or mixing equipment. Pigment pastes virtually does not contain any monomers to harden. Always you to clean stick to stir the color pastes.

If I use high performance gel coat such as Polijel 215 and use standard white pigment from the market, will it cause any problems at the final product ?

Yes, definitely. Outer layer is the most important part in any composite product or part, low quality color pastes doesn’t have UV stabilizers and atmospheric protection additives. Even if you use the highest quality gel coat it will promote yellowing and fading you should prefer Polipigment 01 White for coloring or you can order Polijel 215 white ready to use.

I am planning to increase production volume, which process methods should I consider ?

The volume and the production depends on the product size, shape and technical specifications but briefly.
Low volume production: Open molding (Hand lay-up, Spray-up | Chopped laminating), Vacuum Infusion molding, vacuum bag molding, casting.
Medium volume production : RTM (Resin transfer molding), filament winding, centrifugal casting, wet lay-up compression molding.
High volume production: SMC (Sheet molding compound), BMC (Bulk molding compound), TMC (Thick molding compound), LPMC, Pultrusion, Continuous Lamination.

I am working with open mold (spray-up) laminating, how can I improve the mechanical properties without a major investment ?

You can switch to vacuum bagging or resin infusion methods to reduce entrapped air, compact the laminate and homogenous resin distribution.