1. Both reasons are correct. The Gemara itself does not actually make any mention of "Shalom," but rather one sees from the Gemara merely that it is disrespectful to greet one's friend before greeting Hash-m. However, Rabeinu Yonah (in the back of the Gemara, page 8a of the pages of Rif, DH v'Davka) writes that the prohibition the Gemara mentions applies only when one says "Shalom." This is because Hash-m's name is "Shalom." One may say, however, "good morning" to someone. (Saying "good morning" is permitted only if one sees his friend incidentally; one should not go specially to him to greet him.)

2. If a great miracle happens to a person, he will not be at rest until he has told everyone he knows about the tremendous Chesed that Hash-m did for him by saving him. In fact, he will be angry at anyone who is not interested in listening to him and who tries to change the subject in the middle of his account. Similarly, when we say Hallel we are speaking of miracles that Hash-m has done for the entire Jewish people, and consequently the Gemara mentions the possibility that it may be inappropriate to break off in the middle even to say "Shalom" to one's friend.

3. The way the Gemara explains the verse is as follows: "v'Sheva Yalin" -- "and if he sleeps for seven nights," "Bal Yipaked" -- "without being visited [by a dream]," "Ra" -- "then he is a bad person." Rashi writes that if seven days pass without him being visited from Heaven through a dream, this shows that he is is "Ra" ("bad") and this is why Heaven did not place its Hashgachah Pratis upon the person and visit him.

4. One may talk to one's wife before davening. One may also talk to someone to whom he is giving a ride. The only speaking that is problematic is when one speaks at length. This may be derived from the words of the Shulchan Aruch (Orach Chayim 89:2) who cites an opinion that even if one meets someone incidentally in the street one should say only "good morning" and not "Shalom." The Mishnah Berurah (89:16) writes that even though according to the law one should be permitted to say "Shalom" if one meets someone in an unplanned way, nevertheless the custom is to be stringent. The reason for this stringency is so that one should pay attention to the fact that he should not become busy with anything before praying in the morning. When a person wakes up in the morning he should bear in mind that the most important thing he now has to do is to daven.

We see from here that one should not have a long conversation before davening. If one merely talks to the person to whom he is giving a ride, they will not forget to daven because once theu reach the synagogue, they will finish their conversation and will both daven.

Kol Tuv,

Dovid Bloom