Inactive server instances

By "inactive" you mean shut down? You're still charged for shutdown linodes as they're taking up space and possible RAM (since you could start them up at any time)

Since you're still charged for the inactive linode, the pooled data transfer is the same, it's still there for you to use.

"For instance, if the server is only on for half of the days in a particular month, does this mean that a) the bill will be 50%?, and b) (guessing) that the data transfer allowance is also 50%?"

So a) no b) no. (See a)

Finally (assuming this is correct), what about servers that are powered down for longer periods - e.g. 2 months? Are there charges associated with this?

As per above, you're still charged for them while they're powered down.

Yes, shutdown. Ok thanks, I guess I must have misread a forum comment before!

Although there is some correct things about what you said. When you delete a Linode, you get a prorated refund of the unused time. At the same time, you also get the bandwidth taken for that unused period.

So let's say you purchase a Linode on 1st. You pay $20 and get 200 GB bandwidth. On 15th (out of a 30 days month), you decide you no longer need it, and delete it.

You then get refunded ~$10 to your credit balance, and you still keep 100 GB bandwidth in your transfer pool while 100 GB go away.