1) How difficult is it to add a new gateway?

Depends on your programming ability, but it's not a simple task by any means and requires advanced PHP knowledge. If you've never wrote PHP or only done basic things then it will likely be an improbable and frustrating experience. Not only do you need to be knowledgeable in PHP, but also whatever gateway you're creating needs to have the API that's well documentation. Understanding of latest security coding standards is also highly recommended.

It is worth noting we do not assist with coding custom gateways in any way. We will not review them. We will not debug them. Any issues you may have using a custom gateway should be handled by whoever developed it. We simply can't provide any form of guarantee for code we did not write.

2) Is it possible to hook CBSubs into the user approval process?
Our client wants their users to sign up, then be approved by an administrator, and then receive a link to a page where they can pay, before they become "approved"/"enabled". (sign up -> admin approves -> user pays -> login/use site)

Yes, there is a parameter for payment to be after admin approval. This is done under the Workflows tab of your plans within CBSubs.

3) How does the offline payment option work?
We've taken a look at the CB demo site, but can't quite see how it works. From other posts on the forums, it seems there's a manual step, but what is the user's status until "paid"? (Seemingly, the cbadministrator user on the demo site doesn't have access to see/edit payment methods)

A user purchases with the offline gateway and at the end of the process is given a payment slip. They print the payment slip and mail you a check or cash or I suppose they could do a wire transfer if that's what you're wanting. When the payment is received you pay the basket from within CBSubs > Baskets.

4) Can coupons have on-demand generated/unique codes?
Our client would like to be able to give away coupon codes that give people a free membership, without risking using a blanket "free for all" coupon code. Effectively they want single-use coupon codes, without too much management (they're not interested in creating 100s of coupons/promotions manually). Is this possible?

If you use something to generate the coupon codes or have the coupon code be dependent on a field. So for example I suppose you could use CB Auto Actions to insert a new database row for a coupon and you'd execute the action to generate a coupon code to give out. The alternative is for the coupon code to be set to a field value and adding coupon codes to each users field.

4.5) How does coupon codes that are equal to a CB field work?
We see that it's possible to create a coupon code that is equal to a CB field, which we could then limit to a single use. Would this be a way to do single-use codes? (Looking at chapter 8.4 in the manual, however, it doesn't quite seem to work the way we want it to work)

It pulls the coupon code from a value in the users profile. So you could have a field hold the coupon code that they'd need to supply. Yes, this would make it unique per user. To make it single use per user you can set "Maximum uses per customer (0=unlimited)" to "1". This may work with CB Code Field or CB Query Field to generate coupon codes using PHP or SQL, but have never tried that.