Hi gelaye. Nice to see somebody else doing Japanese - I'm taking Japanese and Korean at SOAS. It is true that there is a fairly high dropout rate amongst Japanese, and more so in Chinese, due to the sheer density of the characters that need to be learnt, as well as the intensity of course. Personally I feel this is due to the low entrance grades of BCC - not that I am stating that any student with lower grades will be in some way inferior, but students with higher grades tend to be able to cope with the pressure of work more easily and are less likely to drop out - and if you are that type of student, then there is no reason that you can't continue studying both. However, I would imagine that one subject will be studied in more depth than the other, most likely Japanese is your primary subject.
As for the gap year, I know that you are placed into one of several prestigious universities as a foreign exchange student, and the nature of your course will be dictated by that Japanese university, with no tuition offered by SOAS in the third year. You might be able to specially arrange another type of year in Japan (e.g. studying a specific element like architecture), but this is probably very rare.
As for the gay scene I don't know much more than you, just that the university is hardly likely to be intolerant of anybody "different" since the sheer diversity of the student body means there is little typical "trend". In any case, I am bisexual and I have little worries about this when I arrive at SOAS.