Absolutely. With around 80 ppm calcium, you could dilute your water half-and-half with store bought distilled and still have 40 ppm calcium, which is just enough for the mash, and cut everything else in half too. One Campden tablet in 10 gallons is more than enough to eliminate chloramine, and will make an insignificant contribution to chloride and sulfate. You'll still need to reduce some of that alkalinity with acid, especially in the sparge which ideally will have no alkalinity. (You could even sparge with straight distilled.) Should get you through this weekend. And you could do the same with RO once you have the unit, if you decide you don't need to build entirely from scratch all for every style.
If you haven't yet, go to the Bru'n Water website and read the Water Knowledge page. It is the most indispensable source of information on brewing water you can find. Well, that and its creator Martin, who's always around to answer questions here on the forum! Download the spreadsheet too if you can run Excel to help with your calculations now and in the future. (The free version is just fine to start with, but I bet eventually you'll want to get the latest updated supporter version. A very helpful tool.)