I would say there would be an element of fixed and variable...

I would say there would be an element of fixed and variable costs. So if waste volume drops materially, then the fixed costs will cause (dis) economies of scale. This is why the waste contracting strategy is so important. Do you run 'spot' do you run 'contracted' or do you run a mix of both? And how does this play out of 20+ years of operation?

Yes - Energy created from the gas will power the operations. I like to think of this as parasitic load.
No - without additional infrastructure (i.e. gas storage or a BESS) it is not possible to release any of the energy gas or electricity at higher prices.
At least to my knowledge DEL hasn't made public any plans to include gas or electricity storage in the business case(s). Of course you could run the analysis of upfront capital costs for the storage options vs the benefits. My view is you are unnecessarily complicating the business case with further unknown variables. But its all possible.

Aussiegusbus - has quoted from previously quoted IRRs from DEL management (probably from the original company prospectus). If these IRR returns can be achieved it would be a fantastic result. Payback periods on infrastructure projects are not quick and will take some years to materialise during operations.