I will answer in reverse order. Arrival at 08:00 for a 10:25 departure is ample time. International and Domestic are in the same terminal.
Question 1 is the more tricky one and you are likely to get different opinions. VA to SQ luggage can be interlined, it has to be done at check in and it involves a small amount of effort from the checkin staff to do this in their system. There will be others that detail VA will not interline but when checking in at SA (ADL-SYD-SIN) luggage has all ways been checked through. In my experience and to answer your question "can VA check luggage straight through" - yes, "will they" - ?
Even if luggage is not checked through, you just collect from arrivals and go back up the stairs to checkin, easy but inconvenient. Also if you are not aware ADL has 2 security check points when flying international.
