I mentioned copper because what you are describing is also like a classic copper meltdown although you make no mention of fish. Like Tom questioned, if you are certain there is no chance to have any copper, don't waste the money.
In regards to your silicone choice to complete this project;
I believe even cured, any silicone that has mold inhibitors is going to cause a problem. My reasoning is rather simple and yet extremely unscientific.
If it's able to inhibit mold after curing, or drying if you will, it's still releasing something in order to accomplish this.
FYI,
I'm not opposed to 100% w/c if the water is prepared ahead, made compatible, and done as a 100% drain and refill. I've left everything in place, including the fish. This is the only way to neutralize a true full kalk over dose as multiple water changes are just about useless in that scenario.
While this is considered a rather drastic measure, and I give plenty of thought before I make a decision to do so, it might be a possible solution your case if you unhook the new sump first.