Japan , especially Tokyo is strict in approving DTVs as the threads show.. Many rejected in Japan are successful second time around elsewhere Laos, Vietnam etc.
Dependents are often approved quicker than the main applicant but it is a busy season now for embassies. Anticipate around 10 business days. Yes, a letter outlining willingness to sponsor is required along with bank/finance details.
It is dependent on your bank manager Similar DTVERS have kept their accounts open, others have had theirs shut..Speak to your bank, hopefully they are understanding.
There is no minimum income requirement. You have regular income so provide the contracts etc for that. Having more than the required 500k is good to show.