Nobody has put it bluntly so i'm going to before you end up with no tank at all. The best advice i can give you is to slow the pace, stop and think. Not that i mean you are stupid, but lets be honest both mistakes (the heat and the improper measure on the glass) are both down to a lapse in common sense and not thinking before acting, AKA rushing ahead. Sorry but it needed saying, i hope you don't take insult, i am not wanting to make you feel bad (we have all made mistakes). Brow beating over, some advice:
Think about how your going to hold it together while it cures, how thin the silicone joint between the glass should be to maintain a good bond, how much time will you have for adjusting the position of the glass (you have these answers already). Have you used silicone much? It can be rather tricky to get a neat bead free hand, you might even be best to mask the tank off and use some solvents to tidy the bead before it sets. Practice might be advisable first for this on some scrap sheet material, it is a display tank after all and messy silicone is ugly. But before you even get to this, you said the silicon is stiffening up on the rest of the tank, does the entire tank need a re-jointing before you set it up long term? Are you certain? Now is the time if it needs doing (if the bond is brittle [not just stiff] or worse, cracking i would rebuild the tank). Also, does the glass on the tank have polished edges and bevels? If so you probably want the glass to match if budget allows, but be warned, some shops also add radius corners when they polish/bevel thick glass (simplifies the machining process from what i gather) which renders it useless for a tank. I literally cannot buy above 1/2" thickness polished glass here without radius corners.
Once you have determined your method, picture doing it in your head a few times first and consider how it might go wrong in practice. If you encounter a problem that you don't know for sure how to deal with while fitting the sheet, stop and ask, you can always start over (silicone is cheap). I have worked with my hands all my life and will say without hesitation that 'don't rush, take your time and think' is the single best piece of advice i have ever had.
On the plus, you now get to use starphire for the back panel too opposed to having one side without. Consider it a happy accident! Good luck with the repair, i'm sure you'll get it sorted with the proper approach / pace and end up with a stunning tank.