...by alone: Está correto?

Never Land,

Nunca vi esta expressão mais gorda. Não recomendo seu uso. Veja como deveria ser, na minha opinião.

They tried to travel by themselves.
They tried to travel alone.

I do not mean to confuse people, however, it is possible to say "by courage alone, by hard work alone, by super human strength alone, by love alone". The meaning is quite different. You are emphasizing that it was only through courage that something was done, it was only through hard work that the house was built, etc.