Yes, you could.

"Súlyos" is perhaps a bit too heavy for apples. A heavy burden can be "súlyos", or a serious illness. Or the cargo on your truck. Or a personality. All in all, overly heavy things. But grammatically it is okay. For "súlyos almák" though, I would think they are made of stone.

No. Because the adjective precedes the noun, it doesn't decline for case or number. So "nehéz" is needed, not "nehezek".