[For him to sail back] is unthinkable.
[For him sailing back] is unthinkable.
Only 1. is grammatical.
When a to-infinitival contains a subject, it also contains the clause subordinator for which appears at the beginning of the clause, right before the subject. Thus, the subject of for him to sail back, is him.
But the subordinator for does not occur with gerund-participial clauses, and hence 2. is ungrammatical.
Your paraphrases are fine. The second is actually the extraposed version of your example 1.
Infinitivals containing a subject, like 1. are quite common:
[For you to accept blame] would be a serious mistake.
[For them to refuse you a visa] was quite outrageous.
All I want is [for us to be reunited].