1) Click on ME and select "Configure Nickname". Set the Label to whatever you'd like displayed and make sure Enabled is checked. The label is allowed to be your station's callsign-SSID, but for backward compatibility with the original documentation, ME will force itself as the label unless you enable your own specifically configured nickname. (Note: Just changing the label without enabling the nickname will have no effect)
2) See http://www.xastir.org/wiki/Search_and_Rescue,_%28SAR%29#Tactical_Callsigns The SARTrack APRS client has recently implemented this specification, but instead of making it an on-demand transmission as it is in xastir, it is done repeatedly on a timer (see http://aprs.fi/?c=message&call=TACTICAL&limit=50). The lastest development version of APRSISCE/32 (see http://aprsisce.wikidot.com/doc:development-version) provides a "Never" option to ignore all future TACTICAL assignments from a given source. It seems that many of the SARTrack users are unaware of the reach of their TACTICAL assigments based on the non-uniqueness and redundancy that many of them are configuring.
3) Not at this time. The internal message retries are fixed as described at http://aprsisce.wikidot.com/message-retries including the second retry set. Five retries is better that some other APRS clients that remember non-acked messages persistently and continue to retry them on every restart for months (or years). If there is a particular message you want to terminate before receiving an ack, do Messages / Pending Messages, select the message from the menu and hit Cancel.
Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32
PS. Responses sometimes come faster via the APRSISCE Yahoo group (http://groups.yahoo.com/neo/groups/APRSISCE/info) as there are multiple people there to answer questions.