Cell phones are not an issue because they're on different frequencies. Brick and stone are serious attenuators at 2.4Ghz. They will literally block the signal completely. This article may help explain:
YES! Since you can't move the walls or drill through them, moving the radios to some approximation of line of sight will be a minimum requirement. A better antenna (or antennas) will also be a huge help.
Replacement antennas are common. Netgear supplies two antennas that my guess would be about 2dBi gain.
In general, everything that's claimed to be 802.11g will talk to each other. However, the exceptions are a problem.
- There are 802.11g enhancements such as Super-G, Turbot-G, and Afterburner, that are not universally compatible.
- Repeaters and WDS bridges are always a problem. The standards are not terribly clear and implimentations vary. Some manufacturers have repeaters that don't even talk to their own hardware due to different chipsets being used. I assume this is what you mean by "propogate" the signal.
Are you thinking of a wireless repeater or WDS bridge? If so you need to make sure the manufactory support the product or at least the chipset used in the FWAG114. I can't tell from the insipid Netgear data sheet if it supports WDS. If so, methinks that's your best bet if you can find a WDS access point to act as a repeater through a hole in the wall. I'm not a big fan of repeaters and WDS bridges, but they do work. Methinks antennas and location are more important and useful. Here's more than you wanted to know about WDS: