1) Can the card be configured to interact directly with a Mac? Or, put another way, is the card usable without connecting to a WiFi network?
The card is the WiFi network. That is, the card presents an ad hoc wifi hotspot, and the connecting device (phone, computer, etc.) connects to the FlashAir's network.
Because it is a flash card, you can always plug it directly into a SD card reader attached to your computer, and transfer files that way as well.
2) Can the card be configured to transfer images as they are taken? And, if so, how quick are the transfer times?
No, there is no "push" capability on the card. However, it does present itself as a standard WebDAV network share. So if you want continuous synchronization, you can manually roll your own with a simple shell script to periodically poll the
http://flashair/DCIM folder, and synchronize its contents (I'd probably use
rsync to do it).
I'm wondering if the card can be used as an alternative to tethering.
Depending on what you find limiting about tethering, it might work as a fair alternative for you. The most limiting factor would be battery life of your camera — the FlashAir is powered from your camera, after all. So when your camera is off while changing batteries, you of course won't have a WiFi connection to the FlashAir card. And you may have to manually reconnect your computer to the FlashAir's WiFi. But perhaps not — you could set the FlashAir's SSID as higher priority network than your main network, so when the FlashAir network comes back, the computer would automatically rejoin it.
The drawback with setting FlashAir has a higher priority network is that, at least at home, you will certainly have faster WiFi speeds connected directly to your home network, as compared to using FlashAir's Internet passthrough feature. Also, if you are security-minded at all, I would presume that the FlashAir has very weak security. And I don't know if using VPN software on your computer or phone is possible while connected to FlashAir, and still access FlashAir's local file storage.