Any day is fine, as long as it shows as received within that month. Earlier is better though in case you experience some type of delay with your transfer, so it doesn't slip into the following month and ruin all your progress.
Only if you're trying to use proof of monthly transfers for your 2nd year retirement extension. Otherwise you can use embassy verified income (if your embassy provides this service) or proof of 800,000 baht in your Thank bank account for the required amount of time.
Embassies don't really have a mechanism to issue a visa for less than the legally mandated duration. The DTV law states that a DTV is valid for 5 years. So it should be issued for 5 years.
For visa needs, you would need your own account. Also joint accounts don't provide any benefits because only one person can ever have that account accessible via the bank's phone application.