Is there an article about the holy grail situation?

Heroic Grails only give you 4 stars. Sorry buddy. A lot of GHB units are good fodder, and you could check their skills by tapping the + icon on their hero page. You don't need to actually use the grails to see their skills. For fodder, Brazen Def/Res is on Kana, and Brazen Atk/Spd is on Linus. Arvis, Saias, and Julius have ploy skills at 4 star. Marisa has Infantry Pulse and Joshua has Close Defense. find out other heroes that you like from their pages.

Umm, you can only get 4 stars from the holy grail summons, there's no 5 star summons. Also, yes, it would be worth it for other skills but that depends on if you really want them.

Yes, BK is the best summon, but H Grails are locked at 4* rarity. There are several others who might be worth summoning for inheritance, like Marisa for Infantry Pulse, HDorcas for HackoLantern or Wary Fighter, Berkut for his Lance if you're desperate, Joshua for Windsweep, etc.

I personally consider BK one of the worse summons unless you don't have him, in which case one copy is not bad.

As units, I favour W.Cecilia, Finn, maybe LA Eliwood, maybe Aversa...Y.Azura if you don't already have her... maybe Joshua... not as convinced about the rest

For fodder, tons of amazing options but infantry pulse and hack-o-lantern are huge. Also good are spd ploy, drive atk, ardent service, any of the brazens and bonds, atk and def ploys, Goodie Boot, and other options which I can't name off the top of my head