It seems to me that if your infallibly assured Magesterium can accept that those who are believers in Jesus Christ as Savior are indeed fellow Christians, then I wonder why "faithful" Roman Catholics have such a hard time with it. The category "Protestant" is only used to designate Christians not of the "Catholic" variety. No one has EVER said Protestantism is a "valid way to heaven" as it is ONLY through faith in the salvific work of Jesus Christ for our sins that ANYBODY can gain eternal life in Heaven. Roman Catholicism is also not a valid way to heaven for the same reason. That you believe non-Catholic denominations are "sects based on lies about the Holy Gospel" proves a biased view that must ignore the very truth of the Holy Gospel derived from the Holy Scriptures.
Don't put words in my mouth please. Protestant means belonging to one of the heretical groups that rose out of the so-called reformation (whether they have some historical claims to the Early Church or not), and condemned by Trent; defining characteristics of Protestantism is the superstitious beliefs in the salvation by faith alone, instruction in the faith by the (truncated by them, too) scripture alone, and various misinterpretations of the nature of the Holy Sacraments.
The heresies of Protestantism endanger the salvation of the Protestants, misrepresent the Christian faith to the non-Christians and are generally bad theology. No union is possible with them; apart from the Anglicans, the Protestants should convert to authentic Christianity individually.
I have no similar objection to several authentic churches that are not in communion with the Roman Church: the Eastern Orthodox, the Copts, the Armenians, and quite a few others. While it is sad that we are separated, we are sister churches nevertheless and I know that one day we will be one again, and it will not require conversion for anyone.