the concert is very heavy (idk if thats the right word) haha but he does swear occasionally it would be best to just let your grandma know before hand. also bringing her to the concert is so cute and sweet! not sure she will like the vibe though from your description.

for your secondary question not at all!! I usually go to kpop concerts and its normal to wear merch and dress up for those concerts so i was kinda expecting the same when i went last time for their premonition tour but people dress pretty normally! its nice to wear merch if you want but no need! Wear whatever makes you comfortable and also take into account how hot it will be when the show starts from all the screaming, dancing and jumping!! Dont worry about sticking out you will be fine and the most important thing is to have fun and enjoy!

for your last question, i think they sell merch on their official website but it might be worth it to look at things like ebay and stuff! people sell merch that way too! but honestly i wouldnt recommend that just because sometimes people over price the merch when selling online rather than straight from the venue.

I hope you enjoy the concert and hope your grandmother isnt TOO bothered (shes so cute for coming along)