Best Network Settings for mostly wireless, sometimes wired?

Question

Folks:
My household --4 desktops Macs, 2 MacBooks, all running 10.4.x or 10.5.x-- lives by using DSL via a Netopia router controlling a LAN, with Cat 5 strung to six desks. No problem there, everything works. Note: for historical reasons --because, mostly, I forget what they were-- all desktops have static IP addresses.
But sometimes one of us wants to work in the house on a MacBook where there is no Cat 5 strung, that is, wirelessly. It's simple enough to set up a wireless server on one of the Airport-equipped desktops and make certain it stays awake.
That works, too, as long as each MacBook is configured with separate Network Preference "Locations" -- one for "Wired" and one for "Wireless", and the wireless configuration is IPv4 using DHCP with IPv6 disabled. Specifically, this requires the user to switch the Location when he/she connects or disconnects a Cat 5 cable.
My questions:
1. Is there a way of configuring the MacBooks so they will work without the requirement switching the Location when removing or connecting a LAN cable?
2. Can this configuration also accommodate what is needed for typical hotspots out there in "the outside world", as in coffee-shops, etc, again without switching the Network Preference Location?
3. Bonus Questions: Would this problem be easier if I modified the wired system configuration to use DHCP instead of fixed IPs? By the way, the Netopia Router will assign addresses in the 192.168.0 to 192.168.15 range, no others.
4. Double Bonus Questions: What are the responses to the above questions, if instead of serving the wireless from one of the desktops, we added a wired/wireless router, say, a D-LINK WBR1310B1 to the LAN? Yes, it's true, we have already tried that. (Our teenager bought and installed it without my...help.) It worked for a while, something unknown changed, and now wiring up the D-LINK brings down the entire LAN. In other words, what are the "issues" of running wired AND wirelessly, and using Airport remotes with a foreign (non-Apple) wireless router?
TIA,
Henry

Answer 1

1. Is there a way of configuring the MacBooks so they will work without the requirement switching the Location when removing or connecting a LAN cable?
Yes, but it's a little freaky (in other words, using the Location menu is the better solution).
To do what you want you need to setup the wireless network on the desktop using a different subnet (e.g. 10.1.x.x) and enable internet sharing (System Preferences -> Sharing).
Then connect the MacBook to the wireless network and make sure it's set for DHCP (the desktop Mac will act as a DHCP server for the wireless network).
Now the kicker is to use MacBook's System Preferences -> Network -> Set Service Order to make sure the ethernet interface is above the AirPort interface. Now the MacBook will use the ethernet interface if it's there and fall back to the AirPort if the ethernet is down.
2. Can this configuration also accommodate what is needed for typical hotspots out there in "the outside world", as in coffee-shops, etc, again without switching the Network Preference Location?
Yes. Most hot-spots will require the use of DHCP on the client, which is the same as how it's set above.
3. Bonus Questions: Would this problem be easier if I modified the wired system configuration to use DHCP instead of fixed IPs?
Yes, but not significantly enough to worry about.
What are the responses to the above questions, if instead of serving the wireless from one of the desktops, we added a wired/wireless router, say, a D-LINK WBR1310B1 to the LAN?
The setup on the client would be the same - the main point is in setting the interface preferences so that the wired ethernet has precedence.
However, a dedicated base station will offer other advantages such as WPA encryption (the Mac-based base station only offers the weaker WEP), and no requirement to leave the desktop Mac running.
From a 'which base station' standpoint, the Apple base stations would be easier to run in an all-Mac environment, but most of the major brands now use web-based interfaces which make them reasonably easy to manage from a Mac. The issue with your D-Link is almost certainly one of misconfiguration rather than incompatibility, but without knowing how it was setup it's hard to advise further.