In Spanish, anything positioned by left hand can be referenced by the word siniestra, which also is an adjective to an evil event or person, very similar to English definition of sinister. This From RAE:
Siniestro, tra. (Del lat. sinister, -tri).
adj. Dicho de una parte o de un sitio: Que está a la mano izquierda.
adj. Avieso y malintencionado.
adj. Infeliz, funesto o aciago. [...]
It might be possible that García Márquez was making an indirect reference, a word game with the siniestra word in the aforementioned phrase.
The relation with izquierda-siniestra has fallen into disuse, being considered a cult word, used in supra-formal. It can see in idiom A diestra y siniestra, referent to Con la derecha y la izquierda, synonym of Sin ton ni son; or A la siniestra del padre, in a religious context, refers to the position occupied by the Holy Spirit.
An example from the Cantar del Mio Cid (2):
Al salir de Vivar, tuvieron la corneja diestra,
y entrando en Burgos, tuviéronla siniestra.
In spanish medieval:
A la exida de Bivar ovieron la corneja diestra
y entrando a Burgos ovieron la siniestra.
In this passage you can see relationship between a favorable prediction (birds flying to the right) while the birds flying on the left were considered a bad omen.
- I don't know any idiom directly derived from this phrase of García Márquez. But the phrase itself derive from a well-know idiom: Más solo que..., a phrase terminating in an exaggeration, like Más solo que loco malo
- No, the word izquierda don't have, accord to RAE, any meaning with solitude or uniqueness.
- I see a union between the loneliness of sinister sites and left hand, using a synonym. I don't know the dialects in all the countries of Latin America, but in some (Chile, Argentina) I can ensure the uso de Más solo que... idiom.
PD: By the way, consider that the left hand and the right hand are alone, separated by a body.