Is fuel in trim tank usable?
Here's a good pilot write up of the A380's fuel systems:

https://captaindave.aero/2017/09/07/the-a380-fuel-system/

Of particular note is this paragraph:


  Towards the end of the flight there are two additional fuel transfers. Any fuel remaining in the trim tank when the time remaining to destination drops below 80 minutes is pumped forward. Similarly, when the time remaining to destination drops below 30 minutes any fuel left in the outer tanks is moved.


So, clearly, on the A380 not only is the fuel usable but it's also a standard operational procedure to utilise it.

I would expect that any fuel trimming is either


Contingent on having more fuel outside of the trim trank. As that fuel depletes, the trim will change, necessitating (or at least allowing) the movement of fuel back away from the trim tank


or


The change in trim is about optimisation, rather than keeping the aircraft within safety tolerances. On a long trip in a large airliner, small changes in trim could have real impacts on efficiency and fuel usage without necessarily impacting control and safety.