Edit: Found relevant article at https://arstechnica.com/tech-policy/2017/02/what-could-happe...
In summary, yes. It is a misdemeanor (subject to a fine of not more than $1,000) to refuse to provide assistance (e.g. translation or decryption). Assuming you refuse, they will likely seize the device and detain you for some unknown period of time.
If by "required" you mean "to get on my flight and avoid getting arrested", then probably yes, it's required.
It's less compelling, but i guess you could have a trusted friend change your passwords for you and require them to later unlock them. (after you get out of detainment 5 days later and fight in court -- instead of the gov't winning, you both lose)
If they cannot perform the search no matter what your excuse is, I seriously doubt they will let that phone, or even you, enter.
If you are a US citizen, they can't really keep you out of the country for this. They can make it hell, but not keep you out. If you are not a citizen, then yes, they can send you right back.