There are not strict rules as there are behaviors that at times can be sexual and at times not. Our current laws to protect people from sexual advances also hurt people from receiving platonic love. For instance, when I was younger, my priest would give us all hugs and we would line up for it, but today that could be questioned.
Also with a woman’s sexuality, she has peeks and valleys. One thing may turn her on one day and not the next. And there are times when even being in the presense of the one you love can mildly arouse you.
The rule is that you should not seek arousal. You shouldn’t surpress your sexuality. For instance, I have a friend I am attracted to and I was in a semi aroused state while in his presense. I could still give him a hug and interact with him fine. I do believe we’re both attracted to each other, but we’ve only known each other for a short while and we’re just not at the point where we’re going to establish some commitment to each other. He did give me a rain check to see Narnia with me though.
My recommendation is to keep your relationship prayer centered. The closer you get to each other, the more tempting sexual expression, even if mild will become. Protect your hearts. Remember that it is your hearts that must be chaste. Its not your sexuality that is evil, it is lust that is evil. Remember the state your relationship is in and that you do not belong to each other, but rather belong to God and that this person is a child of God and a temple of the Holy Spirit. Honor and look at each other’s sexuality as something sacred that you do not wish to descrate. Honor each other’s virginity. The more chaste your heart is, the better you will be able to discern what is right and wrong, but remember stay close to prayer and pray frequently.
In a general opinion, I do not see how sucking on fingers can possibly be anything but a sexual act, so I would refrain from it. I would say that kissing the back of a woman’s hand can be a sign of honor and respect. If you are touching her ear in order to arouse her, than yes don’t do that. If you’re touching her ear to get a better view of new earrings she’s got or helping her to put on an earring, or looking at a sore or bumping it, than that is fine. Or perhaps to kind of tease her with “you’ve got funny/cute ears.” and to kind of point at her ear, but I really see no other situations where one’s ear would be touched unless of course to arouse.