Could you explain what are Mahamudra methods? Is every melting with a budda aspect already a Mahamudra method? Maybe the 3-Light-Meditation of the sixteenth Karmapa is the only method which I would call Mahamudra method.
I suspect that your questions are to a certain degree a matter of which angle you are looking from. By that I mean, depending on how you define things, you will get different answers to your questions.
So, broadly speaking, one could say that all the methods in the Kagyu teachings are Mahamudra methods. That is, all of the practices and teachings of the Kagyus are all designed to allow the realisation of Mahamudra to arise, and all help you progressively move to that realisation. As such, every Kagyu teaching, from the Four Ordinary Foundations (the four thoughts that turn the mind), to the Four Extraordinary foundations (Refuge and Bodhicitta, Vajrasattva etc) are all imbued with the view of Mahamudra and are therefore Mahamudra teachings.
On the other hand, it is more usual to look upon the teachings of the Kagyus as being divided into those which are preparation for Mahamudra, and Mahamudra itself. In that case, the three sets of Foundations - the Four thoughts, Ngondro, and the Four Special Foundations are all preparation for Mahamudra, but *not* actually Mahamudra itself.
From this point of view, all these methods and practices actually prepare the ground in terms of purifying karma, and accumulating merit and wisdom so that the Guru is able with his 'Pointing out instructions' to give you the basis for Mahamudra meditation. Having received this direct pointing out, you then engage in the Enhancement practices which stabilise this realisation, and allow you to develop it to its greatest extent.
As you can see, to an extent it depends on how you look at it. Traditionally at least, you are not even actually meditating until you are on the First Bhumi! Similarly, whatever you are doing, until you have received pointing out instructions and have directly recognised the nature of your mind, you are not yet practicing Mahamudra.
So turning to the specifics of your questions ....
As the 3 lights meditation on HH16 Karmapa is a form of Guru Devotion, it would most usually be included as a variation on one of the Four Extraordinary Foundations (Ngondro), and therefore preceeds Mahamudra, and is not therefore Mahamudra itself.
As far as 'melting with a Buddha aspect' is concerned, it sounds as if you are describing the Completion Stage of a Vajrayana Deity practice. Vajrayana deity practices have a Generation (or Creation) stage, where one manifests the deity concerned, and engages in a variety of methods to develop ones clarity and awareness, and then enters the Completion Stage where you are engaged with formless meditation, and rest in the nature of the mind.
Generally speaking, Deity practices are a key method of the Vajrayana. Mahamudra meditation may have an object, or it may not, and it also may be sutric, vajrayanic, or essense, and finally it may be Shamata Mahamudra or Vipassana Mahamudra, but either way, it generally doesn't involve deity visualisation (though there are exceptions to that generalisation).
Having said all of that, it seems to me that the most important thing as always is that all the teachings are able to take us to Enlightenment, and as such are all equally valuable. Whether something is labelled Mahamudra or not may not be the most important thing. For example, a common Tibetan saying is that 'the Foundations are more profound than the actual practices themselves'.
Another thing worth bearing in mind is that even when a practice isn't really Mahamudra, if you have received some Mahamudra teachings, it's almost inevitable that you will bring some 'flavour' of Mahamudra to that other practice, in the light of any realisation you may have had from those Mahamudra teachings.
I very much hope that may be of some use to you .... and that I managed to answer your questions to some degree?
with best wishes