For wireless client mode, only WEP is supported right now (WPA/WPA2 is supported in ClearOS, but only in AP mode). It looks like Red Hat Enterprise Linux will natively support WPA/WPA2 client mode in RHEL 7 (ClearOS 7). I'm sure there's a way to hack WPA/WPA2 client mode into ClearOS 6, but I don't know how messy that would get.

As for Multiwan, it won't have a problem with the 3G/4G scenario. Given that the network is "barely 3G", you may want to tune the failover settings to be more aggressive. Here's the relevant part of the /etc/syswatch configuration file:

# Retries

# - Number of times we should ping before we call the network "down" 

# - default: 5 



#retries=5





# Ping interval

# - Interval time (in seconds) between test pings

# - default: 60



#interval=60





# Failed interval

# - Time (in seconds) between pings when Internet is down

# - default: 15



#failedinterval=15

You might want to try something like:

retries=3
interval=20
failedinterval=5