I don't have an explanation for 2.
But for 1.:
I know that the hand-off (for a non-active call, just the SIP phone registration) between two different WLANs works thanks to this setting:
Show WLAN availability (choose YES) and Scan for networks (I have it set on Every 5 minutes).
I don't know of a way to insure that that this gets handled properly between WLAN and UMTS.
