Hello IamHajime, welcome to the forum!
1.) If I am to study in Japan, say, for an MA; am I strictly required to have 16 years of education?
I think it should be fine as long as you can explain the gap, i.e. your education system. Students from the Philippines do study here in Japan, so it is of course possible. I think for many universities they say you need a 16 year education
or the equivalent.
2.) What are my chances of getting into an undergraduate program (possibly, all-English like the G30 ones)?
Are there anyone taking any G30 programs here?
I understand your anxieties very well. However, to be honest, questions like "What are my chances?" or "Do I have a strong background?" are ultimately meaningless for two reasons in my opinion. One, unless someone is on the inside it can be hard to judge exactly how the selection committee chooses a candidate. Without statistics to back up the selection habits of a given university, this makes it even harder.
Second, unless it would be a serious burden, you should apply for the universities you wish to attend regardless of whether you think you will be admitted or not. You lose any possible opportunity by not applying in the first place, so naturally your first step should be to apply. Leave the worrying for later
. But, if that answer is not satisfying, in my opinion a B average won't get you rejected.
3.) What are my choices for part-time work? Is it possible to teach English language on a tutorial basis to Japanese students while I, myself, is studying at a university (either undergraduate or graduate studies)?
It is entirely possible and is pretty common among students to do some type of part-time work.
Also, I would like to ask if a Bachelor of Social Science (Law major) + further law studies can become a lawyer?
Thank you!
I'm not really sure, but your language ability will have to be on par with native Japanese. That is to say, you will have to read, understand and produce Japanese on the level of legal documents, something that is hard to do in even one's own native language.