If the CoE states 1 year then its 1 year no more and no less. The exact period is determine when you get the real visa, should be written in there(or probably already written in the CoE - I don't think so tho, I forgot how they look like already). In my case, I arrive, unpack and go to school 2 days later.
And no I don't think you can come early. Unless you got another visa for that, or probably if your country have an arrangement with Japan that you can enter without a visa(don't quote me on this, I have no idea about this)
Or, when you apply your visa with the CoE, you should be able to set the date earlier (100% unsure if this is possible) but the date your visa end will be faster too. (I didn't apply for my first visa, my language school in malaysia helped me apply the visa after getting the CoE) So I'm guessing setting your own date should be possible. Maybe not too far away from the date the school start
Usually in some of my friend case or probably in my case(next year) we are able/should be able to extend our visa to tourist visa without much problem (so says the school I'm attending at) with the reason "joining the graduation ceremony". and I can't really confirm this, but I heard there shouldn't a problem to change your visa to 30days tourist visa after your student visa ends.
If you already got a student visa, theres no need for a round-trip ticket. I don't have one too
p/s: sorry for the too many unconfirmed info, this is the best I can give you based on my experience. Someone else probably can be more clear about this.