RF, Why are you going on this course? There are many other retreat centers available. Why not go elsewhere? As others have pointed out, you will enter the course with an element of dishonesty. You will be lying to get in. And you will be reminded of this fact
throughout the retreat.
I don't say that to make you feel guilty, but those feelings can set the tone of your retreat, especially if its your first one. As you said...
I suppose I don't want to feel like I am fighting Goenka whilst at his retreat.
If you are confident that you can let this guilt go, then by all means, go for it. Attend the retreat, do your thing, and don't worry about it. When all is said and done, its not a big deal! Yet, even knowing that, sometimes the feelings of guilt have a way of seeping in. So, be skillful with yourself.
If I were you, I'd go honest. I was in a similar position to you when I sat my first course. At the time, I too sat with my eyes open, and I watched the breath almost exclusively at the abdomen/chest area. It was tricky to switch styles in the same way it is tricky to do anything new. But I did it, and I'm glad I did it. At some level, I did do some "fighting" with Goenka, but I followed the instructions pretty closely. It also may help you to remember that anapana is only for about 3-4 days. This means that the presence/embodied feeling that you mentioned will still be cultivated for the last 6-7 days of the course as you begin spreading your awareness through your body.
I don't want to miss out on the technique if something magical will happen if I follow the nostrils / eyes closed approach.
I don't know about anything magical, but I have noticed certain reasons for Goenka's instructions. I think the closed-eyes issue is the least significant. You will surely find it weird to close your eyes, but you will get used to it. I now completely prefer it. The only real advantage that I see to keeping your eyes open is alertness. But as you progress, alertness stops being an issue anymore. This will come sooner than you might expect. I definitely find that my awareness of my body is much more subtle and refined when my eyes are closed. But if you want your eyes open, keep 'em open!
However, I suspect that practicing anapana at the nose is more important to Goenka's technique. For one, you are not just training to quiet your mind or calm your awareness (though these certainly are the main reasons for anapana). In Goenka's system, you are also training your mind to focus on a very small area of the body. As your mind sharpens, you learn to focus on a tinier and tinier spot. This becomes a valuable tool later when you are practicing awareness throughout the body. You don't want to focus
only on small areas, but you want to be able to do that sometimes. This teaches you how.
Anapana at the nose also gives you a place to focus where subtle sensations easily arise. The gentle touch of the breath passing over the bottom lip is remarkably similar to the "breath energy/subtle vibrations" that course throughout the body. It is important that you learn to feel those sensations. Feeling the touch of the breath is quite useful for "tuning in" to those sensations.
Also, on Day3/4, there will come a transition point between anapana and vipassana. During this transition, you will be instructed to start giving importance to sensations on the upper lip. If you can sense these sensations, then you give them all your attention (as opposed to the breath). But if you can't feel these sensations, then you simply watch the breath in the same area. This allows you to keep your mind fixed on point, and continue to get concentrated, while at the same time gradually refining your awareness to become more and more subtle.
Perhaps you can shift these instructions to the abdomen. But somehow, it just doesn't quite seem the same. Its hard for me to explain. Just remember that watching the breath at the abdomen certainly has its advantages, but so does watching at the nose. You already know the advantages of the abdomen. I would take three days to see if you can't discover the advantages of something new.
Much metta, whichever choice you make. Have a fruitful retreat!
KN