Can I become a holistic healer without going to med school?

Yes. And you can become an astronaut by getting shot out of a cannon. There are many traditions of healing. You need a medical license to practice medicine in the us. The training is long and hard because people's lives are at risk.
You can, but. You can do it, but you will be more limited in what you can do -- and there will be significant gaps in your understanding. Med school may have its drawbacks, but some schools are bringing in holistic and integrative concepts even with medical students. There are also residencies which incorporate integrative medicine with the "regular" training. The more breadth you have, the more versatile.
Yes. . Because there are no scientifically accepted standards anyone can become a "healer". But if you do you should work closely with a sympathetic MD or DO to make sure you are not missing a severe disease or o muting your patient t .