Re: Loctite 5512 vs. Dow Corning 732 vs GE 2
I would avoid the GE 2. It's not just roots that you should be worried about. It's the fact that the organotins used in the curing process are known to have negative effects in animals.
The other two products, I am not familiar with; however, you said they are "old stock." This alone would make me steer clear. Older silicone will sometimes not cure and it's just not worth it for something that is so cheap and easy to replace. Also, you linked the product data sheets but what you really want to know comes from the MSDS, not the product data sheet. I am not enough of a chemist to know how to read these but maybe someone else can chime in.
As for what is safe to use, there are some known options. If you just need clear, my big box stores still sell GE 1 (which is safe because it doesn't use an organotin catalyst). You can also buy ASI Aquarium Sealant for cheap online and it comes in clear or black. I have had great luck with CR Lawrence black silicone, too, and it is relatively inexpensive. I use clear anywhere it won't be seen and black for anywhere it will :-)
Best of luck,