The shock would be cooling the water temporarily and then it letting it warm up again, hence fluctuating temperature.
Also no need to use the tankwater. Don't u have a water conditioner that removes chlorine and chloramines from tapwater for waterchanges? I'm sure you mentioned it in your original thread, well just use that in tapwater when you add it to the waterbottles.
With both the bottle/icepack method you have to swap the water bottle/ice pack/brick in the tank before it melts = replace it with another frozen one - to maintain cool temperature during the day. You should repeat this throughout the day until the evening. Some aussies placed their axies in the fridge during the hottest months, and also know of at least one axie owner that place their axie in the fridge during the workweek and took them out on the weekends, as they were able to do the icebottle method then.
The icepack/bottle does not instantly make it cold it does it gradually as it melts. As long as the bottle is not too big for the tank - we use a 1.5-2litre coke bottle on our 3 and 4 foot tanks. If you had a smaller tank use a smaller size bottle - 500, 750ml or smaller if needbe. You could also try and get pieces of polystyrene (try the fish shop) and place it round and under your tank = this method in conjunction with the icepack/bottle means the bottle may take longer to melt during the day. One of the aussies suggested it for when they go to work and aren't able to do the constant rotation during the day.
I think at the moment you may be panicking.
If you can't prop your fan up safely, try turning it on and aiming it towards the tank (one of my friends does this and says it helps - he doesn't like taking the lid off); and our temperatures probably get pretty much the same as yours during summer. Also, partially covering your tank or closing the curtains during the day will keep the room cooler, and in turn the tank cooler.