Not much will happen. The land that is chosen to return to your hand is done when the ability from the Garrison resolves. So even if you crack the Wilds, you will still need to return a land (which will probably be that mountain you just looked for)
W may only be paid with white mana. U may only be paid with blue mana. B may only be paid with black mana. R may only be paid with red mana. G may only be paid with green mana. C may only be paid with colorless mana. 1 may be paid with white, blue, black, red, green, or clolorless mana.
To further elaborate on what jskura said; Boros Garrison doesn't target a land. In order to target, the word "target" must be present in the text of the ability. This is why the choice of a land is done on resolution and that choice will be made from any of the lands on the battlefield when the ability resolves. This makes your new mountain a valid choice.
Evolving Wilds
Not much will happen. The land that is chosen to return to your hand is done when the ability from the Garrison resolves. So even if you crack the Wilds, you will still need to return a land (which will probably be that mountain you just looked for)