-Is there still a setting to explore after the ending? Is everything ruined?
The setting is definitely not ruined. We still have a big, lively galaxy.
-Will long-distance superluminal travel still be possible post-Ending? (will Tali or Wrex or Garrus see their homeworlds again? Will everyone starve?)
Galactic civilization will rebuild. The mass relays were not necessary for interstellar flight. Remember, what does it say in the Codex about the speed of ships? That's right, 12 lightyears per (day? hour? minute?). And that's only the cruising speed, not the maximum speed.
People have never needed to research basic FTL improvements before because they have mass relays. With the relays gone, new technology will increase that speed. Additionally, the element zero cores of the dead/controlled Reapers can be used to improve FTL drives. Starflight will continue using conventional FTL.