Boa Sister's Haki is FAR weaker than Luffy right now, everyone should know by now that just Having Known how to use Haki doesnt mean it is equal for all in strength, Boa's Sister knows how to use only Blocking but it is Far weaker than Luffy
Luffy has to and will block Kaido, the clear indication is how Hyogoro stood firmly and told Luffy that he will not budge unless Luffy Blocks BigMom, which clearly means Hyogoro was confident BM could be blocked by Luffy even after he saw BM 2 Shotted Queen.... Luffy being one shotted is not the point here, the point is that Invisible shield was specifically learned for that reason and Luffy was supposed to Block BM if he had learned it.... invisible attacking Ryou on other hand is superior to just Invisible Shield and it infuses own Haki inside object/body in order to cause explosion from inside... Luffy right now should be able to both block and hurt Kaido because that was the reason for training...
and I dont know how your thinking process goes, Luffy already considered 5th emperor and even after training and you clearly saying he will defeat Kaido and BM in collaboration but you are not even ready to at least give him enough credit to be able to block/face a Yonko's attack and still beat them but then next arc suddenly he will be able to not only Block but also defeat Yonko for no good reason of development (while right now we have the exact development for defending which is invisible shield)
Correction, he was supposed to block hakiless amnesiac big mom, hyogoro never saw real big mom, second my point is exactly that. Luffy is a good deal weaker than kaido, even right now. Especially physically, so while yes he may have the barrier haki. It is the same scenario as i told you between the boa sisters and g2. If your ap is strong enough it will go through, and there is no reason to believe kaido cant.
There is reason oda made him learn Future sight, and thats to dodge. And law is there too, and thats to dodge. Again, just because he has to doesnt mean he can.
Only one who considers luffy that is morgans, no one else did not even the WG when naming their bounties, that doesnt matter.
And what do you mean no good reason? He will get stronger haki wise throughout the fight so that by next arc he will be a certified top tier, then you could justify him blocking a yonko or admirals hit with great effort.