Will I get coins again if I disconnect and reconnect a channel?

No. The connection bonus is once per channel.

Connecting & Disconnecting

No. The connection bonus is a one-time reward per channel type, Instagram once and YouTube once.

It is tracked per channel per user, not per handle, so disconnecting and reconnecting, with the same handle or a new one, does not award it again.

Why reconnecting won't repay it

You link Instagram and get 2,000 coins. Later you disconnect and reconnect, or even connect a different Instagram handle. No new bonus: the Instagram connection bonus has already been claimed for your account.