Do you consider this piracy?

For 1) I had the same problem with a XP CD and they consider it piracy. What you should do is to contact support and ask for a backup.

Updated OP.

Even if this was technically piracy, I wouldn't care. You have a licence. You bought it, its yours. You should be able to use it.

So what about the MacGameStore issue...?

Some games on my wishlist are going cheap there, but seemingly not on the WinGameStore, so I was wondering whether I could get them there?

They have to work on Steam nevertheless right?

The first page actually does a good job of covering the topic, so not sure if this comment is necessary :P

Download a pirate version, and use the genuine key (I just want to replay the story anyway)

Legally, this is not considered piracy. For something to legally be considered piracy, one of two factors have to be met [unless there've been recent legal changes I'm not aware of; all laws mentioned are U.S. based]:

1. Duplication
   There are more instances of the object in existence of question following the duplication than there were to begin with, backup storage for personal use aside.
   In other words, in this instance, you'd consider destroying your original disc, as there'd be two copies; however the difference in format abides by the backup clause, wherein the physical copy becomes the backup.
   This was the original legal basis against piracy, and is based in product law.

2. Modification
   It's illegal to bypass security protections in place on an item.
   (Exceptions may apply for personal use.)
   This really won't apply to your situation- it's not illegal to download a modification [you're still applying it for personal use], only to create AND distribute one.
   Basically, it'd be illegal to make AND distribute a NOCD crack [or even a gameplay mod for a game, if it edits existing files], but not to download one.
   If what you're downloading is an unadulterated copy of the game install, then it falls under Duplication rather than Modification. So it'd still be illegal to distribute, but again, not to download if you do own the game. The security laws help protect them in that regard, as you'd be unable to play the game without the CD-Key in that instance anyway.
   
   This is a more modern basis against piracy, where game creators use security laws [that they've heavily helped to shape] in their favor, in conjunction with increased consideration of digital goods as services rather than products [which allows them to cancel legal access if you violate ToS, which a product won't allow you to do]. (ToS of course can't trump law, but it gives them the upper hand in any grey areas.)

tl;dr version: So long as you own the game, you're pretty free to do what you want with it.
Illegalities are primarily based within distribution.

[Though companies keep pushing for stricter laws for digital goods over time, in defiance of basically all preceding consumer/product laws (where right of first sale lets you pretty much do anything you want with a product)]

As far as my personal outlook, it's pretty in line with legality, ignoring the corporate push for increased control, and with more consideration for creative output:
As long as it's for personal use and you're not stealing anything, you ought have the right to utilize what you own in any way you'd like. This includes being able to distribute gameplay mods freely, regardless of what they edit, assuming they don't compromise the security of the game. Since other individuals can't utilize them without owning the game themselves, the whole argument against them is just an abuse of the modification/security laws.

Further thoughts here.

There've been several threads/comments on SG about how MacGameStore steam keys/WinGameStore steam keys aren't system-specific, so presumably you ought be fine. :)

If you know ANYONE with a DVD drive, make an iso of your disk, I use http://download.imgburn.com/SetupImgBurn_2.5.8.0.exe

Then mount the image as a disk using any drive emulator, I use this: http://wincdemu.sysprogs.org/

Not sure if this was mentioned, but some of those games you should be able to activate on GOG too and get access to drm-free copies.

You have a license for the game, you can play the game. It's even on the same platform. This is no piracy. Any argument to the contrary is DRM/corporate brainwash. There, I spoke my mind. :)