In a nutshell, yes...
Any device that sends/receives data via your router is ultimately downstream of your ISP. Consequently, once your phone is connected to your wireless network, it's on your LAN and thus totally divorced from any data restrictions you may have if using 3GS (or any other phone data tarriff via your phone provider).
Not so correct. A wireless access point is just a device that allows you to connect wireless devices to an existing hub, switch or router.
A bridge is a device that enables you split a network into two segments at the physical (MAC address) level, such that network traffic in one does not jam up traffic within the other, and vice versa. I have an old but accurate page that explains it here: http://www.just2good.co.uk/bridgeSwitch.php. However, without knowing a few more fundamentals of networking, this probably won't make much sense, so it's probably not worth pursuing (unless you read all the previous articles first). These days, bridges are not often used, since switches are, in effect, multiport bridges.