Is it permissible for those entrusted with security during Hajj to combine and shorten prayers, and is Friday prayer obligatory upon them if a mosque is available or if there is a neighborhood next to them where Friday prayer is observed?

If you will be staying in Al-Sharae' for more than four days, then pray the full prayer without shortening or combining, because you are considered residents. However, if your intention is to stay for four days or less, or you have not intended a specific duration, then you are considered travelers, and you may shorten and combine prayers. You must perform the Friday prayer in the nearby mosques where it is established.