@jayr.cruz subsequent entrant is another process so that means you should have been granted TSS VISA already before you can apply for them (as long as included sila when the employer submitted their nomination)..
Or it can be a parallel process along with mine? - take note that you may include your dependents together with your TSS application. If you do this, then NO need to apply for subsequent entry. Kaso lang, there is a fee per dependent so you need to pay them..
subsequent entry is usually for people na planning to have their dependents follow them at a later time.. especially that each dependent have a fee as well. We have the same situation, my husband will follow me here in QLD this end of Sept, so Im planning to apply for subsequent 1st week of sept (few weeks ago I have friend applied for subsequent for her dependents and it only took 2 weeks)..
as for your 2nd question: there is no "used by" expiry. Once your VISA is granted, it will indicate the grant and expiry date depende kung how many years granted sayo. You can negotiate wd employer when you actually start work. In my case, I also took almost 2 mos before starting work from the time my VISA was granted..