If stopping being a couch potato and to go outside regularly, they would certainly get away from the trouble.

【翻译】如果他们不再是终日宅在家里（直译：不再是呆在沙发上看电视之人），而是经常到外面走走，他们肯定会摆脱困境的。

此句没错。即便把if 去掉也可以，因为现在分词短语可以表示条件。加上if 是为了更加明确“条件”概念。

if 从句使用现在分词代替 they stopped，这是完全可以的，to go outside regularly 是stopped 的目的状语。原句等于：

If they stopped being a couch potato and (stopped being a couch potato) to go outside regularly, they would certainly get away from the trouble.

句中stop doing sth.不再做某事。stop being...不再是...