Hello,
Why are you worrying? You have the right to stay in Japan for 3 months, so why sweat it? Just enjoy it.
Japanese customs officers do not check your plane tickets; they are just there to see that you donft bring in any illegal or restricted items. They may casually ask if youfre there on business or travelling, so you can tell them youfre doing an internship if you wish, but their main job is to check youfre not importing things that you shouldnft be.
Here is a PDF copy of the front and back of the customs form that they will give you on the plane.
http://www.customs.go.jp/kaisei/youshiki/form_C/C5360-B.pdfBefore you get to the customs desk you will go through immigration and all they will check is that you have correctly filled out the embarkation-disembarkation card for foreigners and have a valid passport. You can see a copy of the card here, front and back.
https://farm6.staticflickr.com/5219/5501454566_8f6c39178f_b.jpgThey will stamp your passport and put in it a digital sticker and part of the card that you filled out (the left-hand top part). They will take your fingerprints (digitally) and a photo of you.
They also donft check plane tickets. The other Customs Declaration form is for the customs officer who you see just before you leave the airport after youfve collected your suitcase, so donft hand it to the immigration officers, as itfs not their job to deal with it.
No one should ask to see your plane ticket if you have a valid passport, and even if they do, you can still stay in Japan for 3 months, so have plenty of time to sort things out. Donft worry, I think youfll love Japan and have a great time, just learn about some of the cultural norms before you arrive.
On a funny note, donft get too stressed out at immigration as they have a thermal camera there to see peoplefs body temperature, and if youfre all hot and bothered, they might think youfve got Ebola or something. ;-)