Can a lone Melchizedek Priesthood holder prepare and bless the Sacrament for himself, when unable to attend church?
The sacrament, baptism, confirmation, naming and blessing of children, priesthood ordination, and grave dedication are all authorized ordinances (see The Family Guidebook). That is, in order to perform any of these ordinances, the priesthood holder must be authorized to do so by someone who holds the keys over it. This is in contrast to consecrating oil, father’s blessings, other blessings of comfort, and anointing and blessing the sick which may be done at the discretion of the priesthood holder. For many members desiring to take the sacrament outside of a formal meeting, they will have to get authorization from their bishop or branch president. I have personally been authorized to administer to my family in limited circumstances and at the time was directed to only do so with a priest or another Melchizedek priesthood holder assisting. Other key-holders have given different instructions based on immediate circumstances according to their prerogative.
Once this authorization is in place, and if the presiding officer so counsels and directs, a lone Melchizedek priesthood holder (or even priest (D&C 20:46)) may prepare and bless the sacrament for himself.
Our military service members are likely to find themselves in such a situation. “As soon as possible, service members should notify their bishop or branch president that they will be leaving for an extended training exercise or will be deploying in response to international tension or wartime operations. The stake or mission president should call a worthy Melchizedek Priesthood holder to preside over the service member group and hold Church services at the deployment site…. Occasionally, a service member may be the only Church member at the deployed location. If this is the case and if the service member holds the priesthood, he should ask his bishop or branch president for authorization to bless and partake of the emblems of the sacrament on his own.” (Supporting Military Families). More information about Service Member Group Leaders and Service Member Groups can be found in Instructions for Priesthood Leaders.