1. You can do a lot with old accounts, but it is more likely an exclusion than the rule, because people as usual have many new accounts and very small amount of old accounts.
2. For young accounts (less than 1 year) Twitter will not allow you to follow/unfollow 3-4k accounts a day for a long time, because it is abnormal activity, which Twitter will not tolerate.
3. Work through API is better from the first glance, but with API, your account is always under the supervision of Twitter. That is why we abandoned this method; therefore, we stopped receiving messages about bans from our customers.