I think you are over-extending on the negligence claim in a matter that’s quite simple, even though it has two elements.
The first is that the upstairs owners have enhanced the value of their property by using common property, and have done so without putting in place the normal ongoing commitments to maintain and repair their changes, or indeed, compensating the strata scheme.
The second issue is that the committee has decided to do nothing about this, a dereliction of their duty of care.
So first of all, you should tell the committee that you want the requisite by-laws for changes to common property to be put in place (including acceptance of ongoing responsibility for repair and maintenance).
Then you want the owners to pay the owners corp compensation based on the standard formula: increase in value of their property (as independently assessed) minus the cost of the actual installation.
And if the committee refuses to put these issues to a general meeting (called at the upstairs owners’ expense), you take them to the tribunal under section 282(2) [Actually it’s 232 (2) – see below] of the Act, seeking orders that the OC passes by-laws as outlined above.