Re point (1) - Yes, in all likelyhood, they would withdraw their offer...
Overall, it is always down to personal choice whether or not to accept an offer below the amount you know you are entitled to - your circumstances might suggest opting to take the sum offered....
But, if their offer is not "
full and final" then there is nothing stopping you from accepting it, and then, once banked, persuing the remainder at a later stage (although I'm sure you would need to discuss that later action with the administrator etc)
Only you can decide what's best.
Good luck