Jesus started the congregation (his body) back in the first century. The message to "the congregations" in Revelation isn't just for "Jehovah's Witnesses" as an org. It's to all "the congregations" that can trace their origin back to that time in the first century. All who claim to be Christian can trace their origin back to that time, not just "Jehovah's Witnesses".
The direction to the seven congregations shows that some congregations have sectarianism. Some have immorality. Some have idolatry.
Jesus said he had some faithful sheep scattered in all those places. He knows where they are.
"The congregation" doesn't just mean "baptized Jehovah's Witnesses." Jesus knows who is one of his sheep. He didn't receive the kingdom in 1914. When he receives the kingdom in heaven, then he will send out his angels and they will remove the weedlike false teachings and the hypocritical people from all the "Christians" and then "at that time the righteous ones will shine like the sun in the Kingdom of their Father."
As regards "non-Christians" and whether they will be "saved"...
If Abraham could be God's friend before being baptized, then a person can be a sheep without being baptized.
"I have other sheep, who are not of this fold..."
A great crowd of "unbelievers" went out of Egypt with the Israelites. They didn't have time to have a long history of "works that befit repentance". They didn't have time to learn everything about Jehovah. At the last minute, when they saw all their idols destroyed and a great tribulation on Egypt, they said "we will go with you people, for we have heard God is with you people." It was enough for Jehovah.