Question
Why is lord Ganesh also called Ekdant, Char Bhuja Dhari?
Answer
Ganesha is depicted having one tooth which has a story that one tooth was taken away by the axe of parashurAma. He has four arms hence he is called as Chaturbhuja also. Also another name is danti.
Secondly ganesha is not the only god who has animal face of body.
There is also Sri varAha, Sri HayagrIva, Sri Narasimha etc.
Regarding your last question, it is a logic based question. However, Ganesha was not a human so it is not proper to compare his neck size with that of the human. So fitting of elephant size head into human body is not applicable. His body was divine so such adjustments may be possible.
The following extract from the "Ganapati Upanishad" * is a specimen of the addresses to Ganesa used by the Ganapatyas: † "Praise to thee, O Ganesa! Thou art manifestly the truth; thou art undoubtedly the Creator, Preserver, and Destroyer, the Supreme Brahma, the eternal Spirit. I speak what is right and true; preserve me therefore, when speaking, listening, giving, possessing, teaching, learning; continually protect me everywhere. By thee was this universe manifested; for thou art earth, water, fire, air and ether. Thou art Brahmā, Vishnu, and Rudra. We acknowledge thy divinity, O Ekadanta! and meditate on thy countenance; enlighten, therefore, our understanding. He who continually meditates upon thy divine form, conceiving it to be with one tooth, four hands, bearing a rat on thy banner, of a red hue, with a large belly, anointed with red perfumes, arrayed in red garments, worshipped with offerings of red flowers, abounding in compassion, the cause of this universe, imperishable, unproduced and unaffected by creation, becomes the most excellent of Yogis. Praise, therefore, be to thee, O Ganapati. Whoever meditates upon this figure of the 'Atharva Siras' (the name of the Upanishad of which the Ganapati forms a part) never will be impeded by difficulties, will be liberated from the five great sins, and all lesser ones; and will acquire riches, the objects of his desires, virtue and final beatitude.
Ganesa has only one tusk, and hence is called Ekadanta. The reason of this is as follows:—Parasurāma, who was a favourite disciple of Siva, went to Kailasa to visit his master. On arriving at the inner apartment, his entrance was opposed by Ganesa, as his father was asleep. Parasurāma nevertheless urged his way, and, after a long dialogue, the two came to blows. Ganesa had at first the advantage, seizing Parasurāma in his trunk, and giving him a twirl that left him sick and senseless. On recovering, Rāma threw his axe at Ganesa, who, recognizing it as his father's weapon—Siva having given it to Parasurāma—received it with all humility upon one of his tusks, which it immediately severed, and hence Ganesa has but one tusk. Pārvati was highly incensed at this, and was about to curse Rāma, when Krishna, of whom he was a worshipper, appeared as a boy and appeased her indignation. Brahmā is said to have promised that her son should be worshipped before the other gods. This result of his contest with Rāma was in consequence of a curse pronounced upon him by the sage Tulasi, with whom he had quarrelled.
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