Is "in [some period]" different from "within [some period]"?
Q1: "I'll finish this job within 5 days" definitely means the job is expected to cost 5 days or less. However, does "I'll finish this job in 5 days" mean exactly the same?
Q2: Can we say, "I'll finish this job in 5 to 10 days" ?
This question is similar to "In 15 minutes" or "15 minutes later"?
Hmm. Within 5 days means, as you say, in 5 days or a shorter period. In 5 days will usually be taken to mean the job will take the whole 5 days and not less.
Yes, I'll finish this job in 5 to 10 days allows you a minimum of 5 days and a maximum of 10 days to finish the job.