Sorry! Could not reply last night. Been tied up for few days.
Dont worry. i dont think water changes performed did any harm to LR/inverts. But dont do it anymore. Too much water changes will constantly upset the system w/o being able to find the reason for pH swing, not that we ever will. Since No fish, why dont you just let the tank stablizes by itself as long as pH does not go lower than 7.8.
For all I know, it could be dieing organisms on the LR (suffered from shipping; not all organisms die at the same time) which may have started to decay.
check for
NH3,
NO2 to see any registering.
Just sit back and monitor and log the results and events w/ dates.
If does not show any NH3 and NO2 and pH has stablized for next week or so, could try adding fish as long as not overload the system all at once.
Not that you should try but have set tank using water, sand, rock, Macroalgae, and fish/invert on same day.
Seen more damages done to fish/nverts when one rush to conclusion.
Hope all goes well.