In chumash, plural means that the people are being addressed as individuals, while singular means they are being addressed as a nation.
The point of the first half is that each individual Jew is present. The verse then adds that the nation's adjuncts - the convert,etc. - are present, hence the whole nation is present. Now the nation as a whole will be bound by covenant. The adjuncts cannot be referred to in singular, since they are neither the parties of the covenant, nor do they belong to each Jew, rather only to the nation as a whole.