"Someone will extract the private key in the next few hours, and then HTTPS will be basically completely broken for all Lenovo users -- anyone will be able to spoof any site to them."
Do you mean the proxy is remote? That is not the impression I have (otherwise having the private key locally makes no sense).
If it's local, then even with the private key extracted, and considering a lot of website force https nowadays, we should still have standard crypto between the lenovo computer and the website. EDIT: As long as the adware checks the website certificate AND doesn't trust it's own self-signed certificate in the store... yeah... a lot of ifs...
Anyway, thanks for the additional details, more helpful than "[...] the certificate allows the software to decrypt secure requests[...]", found in the article...