Hi Oren, welcome to the forum!
If you have Israeli citizenship, you are not eligible for the Mahal program. However, you should still register through the main website for help in choosing a shorter-service program for overseas Israelis.
http://mahal-idf-volunteers.org/about/f ... asresident
You cannot wear contact lenses in a combat unit.
You can start ulpan lessons outside the army whenever you like. However, the IDF ulpan begins only once you have begun your service. So you will not be able to arrive in Israel and start right away.