Yes, your return will be cancelled. This will happen automatically without any human intervention required.
Checking in will not help you (but may deprive someone else of a seat; prices are high, flights are being cancelled, and every seat will likely be needed. But the current operational issues at AMS means you
may find you are currently able to change/cancel your ticket without penalty (depending on how far in advance of your trip we are; but the disruption is likely to persist, and the current timeframe for making such changes will likely be advanced week by week going into the future).
Before you suggest it, having one of your group check you in/show your boarding pass (as well as his own) is almost certain not to work, as it would lead to an incorrect headcount. I would advise you against trying any tricks as this may impact others in your group, and not just you.
Whether your status as group head not showing up to travel will affect the others, I can't say for sure, but I doubt it.
Replacing your current return with a new one-way may end up costing substantially more. But you certainly must somehow change your ticket (in advance) if you want to keep the return portion of your current ticket. Simply failing to board will mean that you are a no-show and your ticket is cancelled. After you factor in the cost of fixing your cancelled KLM ticket, you may find that (even with your cheaper trip back from Australia) you are now paying a lot more for this new journey.
But if it is genuinely cheaper, then why not just get your work to also pay for a "new" AMS-LHR to get you home.
Show them your existing plans, explain to them the issue it has caused you, and then show them how your detour and delay on the way home saves them money, too. Only a curmudgeon would refuse!