Is there a way to load GRP and sprite data in the form of code into slot 1, 2, or 3, so that the game loads everything at the beginning and can go back to that loaded GRP and sprite data without re-loading it?

Depending on how many images you want to keep loaded, you could for example load sprite graphics into GRP3, then display them using SPPAGE 3. You can also use GSAVE and GLOAD to save graphics into an array, which may be more effective than a program slot.