JPcinemamaster: Yes to all, no to all, maybe, sometimes.
I've worked on that area: stock photography, producing (is not an ad, is actually to support and give weight to what I'm about to say).
It depends on the license. Every site, every author, every photographer might have diff terms and can be subject to certain benefits/guidelines the law allows him, be it on his country or internationally. Some terms might apply locally, some people might sell only to some countries (I did discriminate for reasonable reasons).
So the answer to your questions certainly is on each of the author guidelines and license. You might even end up with an special deal depending on your provider. I've done it. Sometimes depending on the case some photographers do give a lot of more freedom, and this includes a written permission for the client to keep for their files and also to keep their minds at easy that they have the right to do this or that.
The problem is when you use "general purpose websites" with license that are not always clear. Back to my previous point, you could ask the authors and keep all the written materials for future reference.
Detailed paperwork. And keep all your receipts and paperwork. Seriously. For life.
Exactly. Remember that little annoying line some people use: "terms might vary from time to time without notice", it can become tricky.
Anyway, as a simple general question, there are royalty free images with specific freedom to PAY only once and use forever and ever as much as YOU want to, except for the right to RESELL. This means, you could buy the image and use it for YOUR client only, not to use it on other pieces of work. In fact you might include a disclosure about it.
The thing about being so picky and clear using and documenting how you use the images, and even including the note on your clients site (somewhere) is because you don't know when somebody might copy that file and then you might be accused of something. Trust me, some photographers do check upon their images and hunt copy pasters.
Sometimes you can get better deals with respectable, humble and nice photographers who happen to have nice work and will be nice to you as a person and giving you a special deal instead of tight licenses.
So in short, your answer is on your providers terms.