No. Excommunication is a form of church discipline that is used in cases where persons have committed serious sins. By simply stopping church attendance and participation, you are not risking excommunication.

However, if the person wishes to have their name removed from the records of the church, a formal letter has to be submitted to Church Headquarters. There are plenty of websites on the net to give you examples and all that you will need. This is done usually by people who wish to cut all ties with the Church (that includes people calling and dropping by every so often) but have not committed any serious sin.

Yes and no. Excommunication is literally "the ending of fellowship." Thus, someone who leaves the Church and asks for their records to be removed is excommunicating themselves. However, the word "excommunication" has improperly taken a subjective meaning implying punishment, so the term is no longer used to describe the process of someone deciding to leave the Church. In fact, while excommunication can be a result of disciplinary action, excommunication in such a case is not considered a punishment, but instead the first step in a second chance, and nearly all of those who have been excommunicated may attend meetings and work their way back to membership and even the Temple, if they so choose. Those who leave the Church and request their records to be removed may likewise attend meetings, if they are not disruptive, and if they choose to return to the Church (many do), that path is open for them unless other factors prevent it. If you leave the Church you are not automatically excommunicated. If you ask for your records to be removed, the process is not called excommunication, but the result is the same, you lose all of the blessings of membership. So no, you are not excommunicated if you leave. The effect might be the same, if you choose for it to be.

If they just decide to not attend Church then no, they cannot be excommunicated. But if they stop attending because they have for example, run away with another woman, run away to or create another church/religion considered to be an apostate group, because someone found out they were fornicating, were engaged in adultery, had killed someone, committed a rape, were engaged in pedophillia, ran away and started publishing anti-Mormon literature, was considered to be apostate, then the Church can excommunicate them formally/consider excommunication through disciplinary procedure even if the member fails or is unable (eg in prison) to show up for the 'trial' known as a disciplinary council.