Bitmask et al, you are right on all your assessments. Indeed you could say quite frankly that Bantu people are very able to discern each other across different tribes, all you need is to spend a little time understanding the etymology and linguistic style/consonants and you will be good to go. By this i mean, certain words in Bukusu (similarly other luhya sub-tribes) could be spoken in a stressed manner to effect the meaning and it could mean a slightly different word in a different sub-tribe (e.g. Omuloosi ... affectionately means a lady/my wife in ki-Bukusu, but in ki-Marachi she may interpret it as a witch; so this one time a long time ago my uncle was on phone with someone speaking of his omuloosi (mama wa nyumba) and she interpreted it to mean (got it) that she had been called a witch (Omulosi)! Wazees had to be called in to calm this kerfuffle!
On the whole, the Luhya sub-tribes do have many similarities that naturally coalesced to the grouping Luhya. So I will not pin-point any qualifying marks to being a Luhya.
Luhya is a thing. They are a closely related group of people culturally and genetically. Politically they are yet to find a unifying force. So while there was nothing known as Luhya until the 30s, lumping them together is far from arbitrary.
Linguistically, there are definitely languages that can be grouped together as Luhya. Intelligibility can be high to very low. Linguists think there are two main branches, some would say three. Luhya-proper(Busia, Kakamega folks), Maragoli (and related tribes in Vihiga like Tiriki), and Bukusu(Tachoni). Bukusu are also close to Bagisu in Uganda (linguistically and culturally).
From what I have gathered, Bukusu and Luhya-proper tend to have high levels of intelligibility (Pragmatic correct me if I am wrong), while Bukusu and Maragoli can barely understand each other. But Bukusu and Maragoli are still demonstrably Luhya languages. Did they see themselves as a single group, say 70 years ago? Probably not. But now they do. That is for the most part what matters.
The Luhyas have had politically unifying figure in Masinde Muliro and in recent times, Wamalwa Kijana. No small matter that both these were Bukusu... and it goes without saying that the Bukusu being the largest ethnic group in the Luhya community feel that they must be the ones to provide the leader. There is a running joke with the Bukusu that they used to raid the Maragoli and steal their girls (so a lot of Maragoli girls bore many Bukusu blood
). A small whisper though, there is really no pure "blue blood" Bukusu (who is Bukusu??), even Masinde Muliro and later Kijana Wamalwa are said to have had some other blood, Banyala or such....
It doesn't mean that a Maragoli (Mudavadi) or Wanga (Oparanya) cannot ascent to Luhya leader.
Those two gentlemen didn't wait to be gifted the Luhya leader status.... they went for it and won the respect of their fellow leaders and Luhya hoi polloi. They were towering high among equals. In both of them, there were many equally able figures in Luhya land at that time, but they thought these to be the leaders among them. Currently, there is hardly such towering figures and even the Oparanyas have not reached such level.
Now to Mudavadi.... Mudavadi waits to be crowned/coronated and sits there bumbling around and always saying that Raila is disuniting the Luhyas (forgetting it is Raila who foisted him to the highest Luhya pedestal he ever reached!). And he and Wetangula even in their combined tag team cannot excite the Luhya masses enough. Yet these two, if they had the oomph constitute 75% of the Luhya vote! Mudavadi simply lacks discipline and gravitas to lead a force to achieve taking power. He is a coward! He is too lazy and a privileged politician who has never had to sweat to arrive at where he is.
They will be scattered to the winds and soon you will see either Wetangula or Mudavadi abandon the other and line up on the Loyalty Pledge chorus!
What they lack is a politically unifying figure. MDVD has had the best chance in the past few decades to pull them together. But I think he faces resistance from the Bukusu leaning spectrum of the Luhya. Weta has had the populous Bukusu to back him, but he lacks the gravitas to hold even this group by itself together. People like Oparanya and Atwoli could easily have been unifying figures if they came from the bigger groups like Bukusu or Maragoli.
And finally as a parting shot.... forget Njuri Ncheke's low opinion of the Bukusu... the Bukusu are the Kings of the Luhyas and FYI Njuri; they are warriors, but we don't go around announcing our "tigritude"! Read your history a little deeper.