[Nyororin]

As an actual linguist, I`ll do my best to answer.

In this case (Rachel is vs. Rachel`s) the "is" is written as a disambiguation. What MMM said is essentially correct - A noun followed by 's represents possession in almost all cases ("it`s" being an exception). In spoken context, you`re not likely to misinterpret the meaning, but in written form possibly without context - it`s a possibility.

Yes, language changes over time, but I have yet to see a real example of a change that made language more difficult to understand being accepted. That is what allowing 's to be "is" when attached to anything would do.