Best advice I can possibly give you is to cut bait and walk away. She's in a cult that controls just about every aspect of her life. Even if she seems like she doesn't believe/do all of it, the indoctrination can lay dormant beneath the surface for years waiting for some life event (often death or having kids) to trigger renewed cult involvement. There are many "unbelieving mates" on this forum that are here because they married an inactive JW that rekindled their cult involvement in this way.
Since many folks in your position tend to brush off the above advice, your next best hope is to read Combating Cult Mind Control by Steven Hassan to get a better idea of what you're dealing with and try to help her to wake up from the cult indoctrination. Once you've read through that you can use sites like this one and jwfacts.com to learn about the cult and introduce information that may help her to see it for what it is. This is likely to be a large investment for you and is in no way guaranteed to work. She's already shown that she'll put the cult and her mother ahead of you, so there's no reason to believe that she'd change course as you try to expose the cult. Most likely she'll become very cold and push you away, which is why my best advice is to move on.
In answer to the question in your title - JWs are STRONGLY discouraged from dating any non-JWs ("worldly" people) and will often face pretty harsh sanctions for doing so. If she's having sex with you (or anyone she's not married to) and that is discovered, she will be "disfellowshipped" and shunned by all JWs. Since she was raised as a JW, that means that she will probably be shunned by just about everyone that she's ever had a close relationship with (since she's dating you she's clearly a little more liberal about outside relationships, but I would hazard the guess that you're the exception, not the rule). If she is/becomes "inactive" (this means she stops preaching door-to-door) and stops going to meetings her dating you will likely result in shunning by most/all JWs even without her being officially disfellowshipped - they will assume she's having sex and that she needs to be disfellowshipped but just hasn't gotten caught.
As a side note - her belief in the age of the earth is not evidence of her straying from the JW stance - they have somewhat informally accepted the earth's age in recent literature (just doing it in such a way so as not to turn off old-timers that remember when the earth was supposed to be ~50,000 years old). Evolution is also a kinda weak point for her to be liberal on - most JWs don't put enough thoughts into their beliefs to realize that if evolution is true it completely unravels their doctrine. You may also be mistaking her acceptance of so-called "microevolution" for a real understanding of the issues. JWs believe in the global flood of Noah's day as literal and believe that afterward god introduced variation in the species to get from the limited population on the ark to what we have now. My point in saying this, though, is that she may seem somewhat reasonable about certain issues, but if you demonstrate that they show the cult doctrine to be false then she will almost certainly recant what she's said and burrow deeper into the cult.
I wish you the best of luck, whichever course you decide to take.
