1. Reason for not having any celebrations yet? Can I just explain briefly as I stated above or will I need really detailed explanations?
Just explain as you did above. If you had any type of celebration - even just a dinner at a restaurant with her parents - you can include it.
2. In the 'Sponsor Evaluation' section, will it look back if I check the 'Unemployed' box and write my explanation that I had just returned to Canada so I do not have a job yet?
It is better to have a job, but not 100% necessary. Maybe by the time you get the application forms filled out and all evidence gathered, you will have a job. The visa officer will want to see evidence that you can support the applicant, so you can include other things as evidence: proof of savings and property owned (yours and the applicant's), evidence that shows you and the applicant will have no trouble getting a job, an explanation of your plans for your life together in Canada: where you will live, what job you (will) have, what kind of work the applicant hopes to get, etc.
3. What do I do about the 'Option C Printout' since I haven't been working in Canada for the last while?
You can write an explanation of why you don't have it. It is good to then include evidence of the job you had in Japan. Try to include the Japanese equivalent of what they ask for in the form.
4. Is there anything I have to get signed from my current Japanese employers?
You don't need to, but it won't hurt to include a letter from him detailing your job details, including salary. Or you could include a copy of your contract.