I'm afraid I can't help much as incense is outside my area of expertise, however in a liquid fragrance you would normally use any of a number of aroma chemicals to improve diffusion: hedione is a classic of the kind.
Solubility isn't an easy one either - DEP is a widely used solvent for aroma chemicals so many will be soluble in it anyway. If something isn't then you might try a co-solvent such as DPM (dipropylene glycol mono methyl ether) however I've no idea what that might do in the context of incense.
Hopefully someone with better chemistry knowledge can help . . .