@Flyingpantsu She was not being stupid here, her main purpose was attracting the 2 girls attention so Gramie would attack them from behind. And as you saw, they were successful
"Second stupid thing : Myura and Rin could have acted during all the time the baddie was describing her shiny new item but the latter waited politely until she was done before trying to act at close range against a known close ranged fighter instead of putting some space between them and the former simply didn't do anything at all until she got punched in the guts..."
I'm sorry but that wasn't possible, he explained the damn thing in lass than a second and attack.
Just watch the page:
https://mangadex.org/chapter/699626/25
See here, she ended the explanation in the same panel she removed the damned thing, this means that there wasn't enough time for them to get out of their shock from having someone showup literaly behind their backs. Gramie attacked while they couldn't react. and the panel disposition shows that it all happened within a second.
Frankly for a generic series such as this, this chapter here was actually good, with enemies using a believable tactic to take down enemies that are stronger than them.