The date of Arassuil's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
2
There is no clear explanation of Arassuil's full name. There's an evident similarity to suilad 'greetings', so Arassuil may mean something like 'he who greets royally', but there's no direct evidence to support that connection.