Vincent, thanks so much for bringing all this up! I read your post then took my dog for a walk and I was very content reflecting on it. This, then, is a long response
I've had lots of people in my classes, and most I don't know the religious/spiritual faith of. In a general classes where I've chosen philosophy or spiritual themes I remind people that they can take however much or little of what I say with them. I give them a free pass to ignore whatever doesn't resonate. After all, I can only speak from my individual experience of spirituality and I would resent being asked to take on anyone else's view in the context of a yoga class. Of course, I can say that clearly because I have been in positions where it was, wrongly, assumed that all present were on the same page, as it were. That is not the environment I wish to create or be a part of.
You are already finding that there are parallels between your beliefs and some of the concepts in yoga. I find that from a yoga standpoint I am more able to connect and relate to organized religions, so it goes both ways. But whether one can harm the practice of the other? I think that fear is all in how you perceive things.
You say that 'sometimes religious elements seem to be present in class' - what in particular do you mean? If you could say what potentially alarms you, you may be able to get a clearer answer to your feeling of being stuck in the middle. For example, I could take that to mean how Frankincense incense is a prime ingredient in Catholic incense, and it does show up in the east and in yoga incense. All activities have a beginning, middle, and end - and whether a spiritual activity or a religious one (I do find there is a distinction between them) there are common elements. I do not think yoga is a religion - or, if it is, it is like the blanket term 'Christian' and gives no indication of the variety of practices that can be found.
I will say, having been approached and questioned by Christians as to the meaning of some things in class, the following things that I've found helpful to find commonality in or various understanding of:
OM is the Hindu concept that maintains that the Universe arose from a vibration; quantum physics now states that rather than matter and energy theres is only energy, and that energy, being a wave, is vibratory and this supports the Universal vibration theory of OM; from New International Version (©1984) we have the translation of John 1:1: In the beginning was the Word, and the Word was with God, and the Word was God. and I've always seen this to be a similar idea of vibration.
Namaste, while it can have many long meanings, can be simply translated as "I honor the Spirit in you which is also in me." For those who see delineation and separation between individuals as well as with god, a dualistic perspective, this might be taken to mean that you acknowledge the other person as a child of god, the same as you. From a nondual perspective there would be the idea that essentially there is only one. But it's also simply the hello and goodbye of India, Tibet, Nepal, and can be used in that way to thank those present at the class.
You are right to feel that the yamas and niyamas have foundational similarities with your faith. Do they offer a 'Hindu version' of what you have already from your Catholic faith? Probably not. But they do offer a different way of thinking about your behaviour, one that is not morally based, that is tells you what you will be punished for doing, but ethically predicated and asks you to look at the way you do a larger number of things so that the principle of good action comes to bear on a larger realm of your life. In that way it's not just that we refrain from doing someting, it's that we work to be in integrity in all our actions.
You're alright with asana (makes you feel better) and breath (no conflict as you say) but stumped with the last few limbs. I would put them to you this way: if you recite the Hail Mary or Our Father (both which can be seen as mantra practice, in fact there have been studies on the Hail Mary and Gayatris that show they induce the same brain patterns) you are already working to foster withdrawal of senses (pratyahara), concentration (dharana) and meditation (dyana) on not only the rosary and the phrases, but the qualities that those inspire in you. Your meditation just might be more along the traditional concepts of prayer. Samadhi, for you, might be the attainment of heaven after death. There are lots of differing explanations of samadhi and nirvana, and I believe that in the end, it is an individual who creates the meaning of these in their life.
In yoga classes I feel we are rarely given an opportunity to practice these last four limbs, perhaps meditation a bit. But as all my meditation teachers have taught me, from yoga, buddhist, and other traditions: our mind will only be empty if we are dead; that's not the goal really. To say that meditation empties the mind isn't actually a proper description. More to the point we will be able to remain unreactive to the busyiness in our minds, the judging, the future planning, the memory re-inactments, etc. We will develop better inner discernment that leads to more appropriate outer actions with those around us. We will be able to sit with the chaos but not be intangled with it: be in our inner mind's world, but not of it. Does that make sense?
As for trances...I think the only thing you are likely to find with yoga practice is that you move into the present moment, which is like entering the well talked of 'zone' - it's not an altered state, it's a fully present state.
And, finally, about the becoming one with God in yoga: I think this varies more than people believe. Patanajali's 8 Limbs and the Upanishads for instance, both classic texts, are at odds about what this will mean. For myself, I do believe in a nondual situation, but I find that often in yoga philosophy what is espoused is a dual one. If anything, all people are hoping to be better able to hear their god(s) and follow the guidance and advice given by them, I think yoga can absolutely help this ability.
In the end, it is my desire in teaching that people use yoga as a tool to help them establish and/or maintain their spiritual or religious beliefs, whatever those are. For me that means that divorcing the poses from the other limbs creates an intense stretching class that is not yoga (there are, after all, only so many ways the body can move, yoga has no exclusivity over body movements, just special names and sequencing theories). But would I be able to tell if anyone came to my class and ignored everything I said except the asana portion? No. So, there are probably people doing just that, getting a good, safe stretch and letting the rest fall away. I love that idea! In the end, they might choose not to come to my classes but find another teacher, who isn't so 'woo woo', and that too is great.
There are plenty of styles, and even more teachers, for everyone. One of the studios I trained at has almost none of the 'normal' yoga trappings on display. They use tingsha bells or a singing bowl to bring you out of relaxation at the end, and say 'namaste' but aside from that it's a totally neutral space, which is what the owner wanted to offer people.
So! That was long winded! Hope something sparked in you - and looking forward to continuing the conversation!