TL;DR: Fly. Attempting to drive a rental car across the border as a tourist is a non-starter. You won't get the required NOC from the agency, and dealing with insurance (Orange Card) at the border is a hassle you don't need. Your instinct about the bus is right—it's a 6+ hour slog with a tedious border crossing where you have to haul bags for scanning.

Since you're seeing cheap cash fares, just book them. Wizz Air (5W) from AUH or Air Arabia (G9) from SHJ are consistently cheap. Burning miles on these short hops usually yields terrible CPM unless it's a last-minute peak travel date.

For Salalah (SLL): Do not drive. It’s a 10-hour drive through empty desert. Fly Oman Air (WY) or SalamAir (OV). Regarding redemptions: The only real "sweet spot" here is using BA Avios for MCT-SLL (Zone 1). It’s 6k Avios in Y or 12.5k in J. If revenue tickets spike >$120, use points; otherwise, pay cash.