Now while I agree that it seems highly unlikely that the Celtic tribes could penetrate, let alone conquer, the Argead Balkans (after all, they did send emissaries to Alexander's court with the believed motivation of seeing how strong/vulnerable the Empire was, and deemed it too strong to attack), let's not say it is impossible.
Let's recall that the Celtic army of some 300,000 men, assuming the population was similar 50 years earlier than the OTL invasion, pouring south along the Danube is no laughing matter. If the tribal leaders are smart, they'll stick together and avoid fracture, which means that at any given time, they have 3x times the army of Achaemenid Persia, and 6x the size of Alexander's army in Asia. Regardless of individual worth, 300,000 Celtic warriors, many of whom were on chariot, will overpower with sheer numbers whatever force is mustered against them. Initially at least.
And that's where the Celt's have some issues. Initially, they will enter Argead territory like a sledgehammer pounding through plaster. But their motivation largely isn't to conquer, it's to raid and plunder. If they remain centralized, head for the big cities and crush them one by one like an Eastern army would, there'd be no stopping them. But the Celts didn't function that way, and would split up into smaller units to divide the wealth laid out for them. After crushing Antipater (yes, I think that is actually a likely outcome), it will take some time for the armies further east to respond, which will leave the Balkans largely defenseless until reinforcements from across the sea arrive to drive them back one army at a time. And when you have that many armies, you begin to realize that whatever Macedonian force that arrives to push them back will have to worry a lot about getting whittled away bit by bit with each battle. How many battles will they win before they don't have enough men? And will that have been enough for the next batch of Macedonian reinforcements to arrive and finish the clean-up?
The much easier solution would be to bargain with the Celts. Alexander might want to use them, like Nicodemus, and settle them further east. You could end up with a Galatia in Bactria or Arabia.
EDIT: My high estimation of there number is due to the use of Trimarchesia. For example, an ancient writer might say there were 100,000 warriors, perhaps a high estimate, but with the implementation of Trimarchesia, where two more "squires" were ready to replace a warrior in battle, the number triples.