Why would LE need to remove TH's DNA from her own car key? That makes no sense. TH DNA on her own key would be expected. If SA's DNA is on the key in addition, that's compelling. One person's DNA doesn't turn into someone else's.
Why would someone want TH's key to not have TH's DNA on it?
As mentioned by Missy, it isn't about LE wanted to remove Halbach's DNA from the key. It's about LE wanting to remove their own possible DNA on it. And it wouldn't be difficult to then plant Avery's DNA on it. If I recall correctly Colborn and Lenk were both processing the bathroom just prior to the key showing up. The bathroom would be ripe with sources for Avery's DNA... Toothbrush, toilet, any handles or knobs, sweaty socks, armpit of dirty shirt, etc.