The reason Excede is approved for ear administration has to do with the meat withdrawl period.
As the ears are removed at slaughter and not used in any way for human consumption, they can have a shorter w/d date then they would for a SQ approved product given in the neck.
It is no less effective given SQ in the neck BUT it is off-label and you would need your veterinarian's blessing to do so.
The drug has a longer duration than Excenel/Naxel because of the carrier the drug is suspended in and the drug concentration is 4x higher.
The reason you have to avoid arterial injection is that the drug bolus would go straight to the brain. Lights out.