Make sure you are using the latest CFW of whatever you're using. Also deleting the ticket would make certain games unplayable because the ticket is needed to launch it. If its an .XCI convert, it won't have a ticket to delete however.
Just change your boot.dat to the one used 2.5.1
What is your current firmware? Which version of SX OS are you using?
EmuNAND has a few advantages. For one thing, it can let you enjoy the benefits of being on a higher firmware without updating SysNAND or dealing with AutoRCM. Another advantage is that EmuNAND can act as a sort of "ban prevention" tool. If you do all CFW and homebrew in EmuNAND but keep all legitimate online play in SysNAND, your chances of being banned reduce dramatically since SysNAND and EmuNAND don't share any information whatsoever. The last advantage of EmuNAND is that if EmuNAND gets bricked, it won't brick your SysNAND. The cons of course is that you will need the required space to install EmuNAND whether it be the internal memory or on the SD card. And of course, the only way to update EmuNAND is with ChoiDujourNX because updating online will flag the whole console for a ban, both SysNAND and EmuNAND.
Maybe try removing the data with Goldleaf? If that doesn't work or you don't want to try it, the only other option is a factory reset. Make sure you backup any save data and anything else you want to preserve before going that route.
You don't have to delete anything off of the SD card since there is no evidence that Nintendo scans the SD card.