What’s the difference between iTunes Match and Apple Music

Yes, you can add the second Mac once the first has completed. While one is currently uploading, I think the others can only be linked and start viewing.

I haven't tested it (Match is not available for my country) but from the details it looks like you should be able to delete the songs locally and rely on the cloud storage to stream:

Once your music is in iCloud, you can stream it to any of your devices. Just browse the complete list of all your music stored in the cloud. To listen to a song, tap the iCloud icon next to it and your song starts playing. You can store up to 25,000 songs in iCloud (more if songs are purchased from the iTunes Store), but only what you want to play is stored on your device. So you have immediate access to a huge music library without taking up storage space.

Have a look at this Mac Rumors discussion, it seems that after a song is matched and you try to delete it, a copy will be kept on the HDD in order to play it faster and not redownload it.

Edit: I assume you're aware of the perils of solely relying on cloud storage for your music library: it will only work while connected to the cloud, availability is subject to Apple's benevolence (with little warranty) and terms of use might change anytime.