According to my understanding,
a) Buddha-Sāsana is the fourfold community of Bhikkhus, Bhikkhunis, Upāsakas and Upāsikās. They started with the Buddha and at some future point will end.
b) Buddha-Dharma is simply the doctrine of the Buddha. Here the word Dharma is used in a more general sense to mean 'doctrine' or 'teaching'. Hence, Buddha-Dharma, Hindu-Dharma, etc(Hindus call their religion Sanātana Dharma, i.e. 'the eternal doctrine').
I don't think anybody's saying a modern Buddhist has no authority - nobody's being forced to accept anything against their will. One will prefer to side with the traditional understanding(s) more or less depending on how reliable they think the tradition is, based on their own understanding and the practical results they have obtained.
But there is no way of completely doing away with tradition: the way we understand the Pāli language, for example, comes from tradition. If one can't read Pāli, then one is at the mercy of tradition even more (since all translations are interpretations too), whether it's Bhikkhu Bodhi's traditional understanding, or whether it's Ven. Thanissaro's, or Stephen Batchelor's. Tradition is like a tree, with some branches closer to the trunk (e.g. Pā Auk Sayādāw), with some far away from it(e.g. John Peacock), and some in between(e.g. Ven. Ñāṇānanda). I don't think anyone has a perfect alignment with the trunk.
It's also a matter of how much time and energy we can spend. If there's infinite time, then we can try down every avenue, try out every little pet theory, etc. But we can't, so we have to make judgements, make a stand at some point, and decide how much we want to stick with this or that tradition. Most of us don't have time to try out Ven. Brahmavamso's jhāna teachings, and then try Ven. Gunaratana's jhāna teachings, and then Brasington's, etc. So we have to make a choice and stick with it, for some time at least. It's also based on the goal: is your goal a stress free life, or is it Sōtāpatti? If the goal is Sōtāpatti, and if you think the Mahāsi tradition is capable of delivering it, then any other concerns about that tradition is going to look very trivial to you.
"Delighting in existence, O monks, are gods and men; they are attached to existence, they revel in existence. When the Dhamma for the cessation of existence is being preached to them, their minds do not leap towards it, do not get pleased with it, do not get settled in it, do not find confidence in it. That is how, monks, some lag behind."
- It. p 43