Why would Lannister soldiers lie to Jaime about the Blackfish?

I don't get introducing these characters to so easily discard them.

Same with the Septon last week. He seemed to be based on Septon Meribald, who delivered one of the best speeches in the series so far. It was pretty much the defining discussion of the series on the topics it covered. Such a wasted opportunity, with such a top flight actor...

Also, this is out of place, but does anyone have a clue as to the rumor Qyburn was discussing with Cersei? Gotta believe it's about the High Sparrow.

Edit - One more thing: D&D really ****** up the storyline here. Ellaria Sand and the rest of Dorne straight murdered Cersei's daughter... so naturally, she sends Jaime and the Lannister army... to Riverrun? Dafuq? And then another awful scene with Tyrion, grey worm, etc... The writing has been absolute **** since they went off GRRMs script. Cannot make excuses anymore.