Maarten D. de Jong
1) May I place an influence marker in a province with foreign land units?
No, unless it concerns your own home province, or the units are ships which have not been yet moved into the sea. (Land and sea units do not ever influence each other.)
2) May I convert the capital province of a neutral civilization?
Yes. Neutral civs may be fully overrun.
3) "Nebuchadnezzar: When your legions sack a province, take one Commodity card for the destroyed caravan, or one Tax card for the destroyed city." If I destroy a caravan may I take a Commodity card of my choice or must it be of the commodity type the caravan produced?
The latter option.
4) May I take an "occupy action" (that is: sack or occupy or convert a province) in a province more than once during my turn in the Military Phase?
This is a common question. The answer is no, unless you have chosen occupation as their special action---you can regroup to your heart's content. Strictly speaking you can always 'undo' a sacking or conversion and do something else instead, though. But at the end of your turn, you may have chosen but one of the three possibilities for every province where you gained the right to do so.
E.g. I have 3 legions adjacent to a province with 1 influence marker, 1 legion and 2 caravans of another player. I move 2 legions into the province and attack. The other legion is killed and I destroy a caravan. After that I immediately move another legion in and declare another attack. According to the rules (page 9) I win the attack automatically and thus I can take an occupy action again (e.g. destroy another caravan). Is this really allowed?
No. You sacked the caravan---that was your single special action for this particular province in this particular turn.
5) If I occupy buildings or the influence marker in a province and in the next round another player's legions are also in the province and I fail to kill them all, must I remove my legions from the occupied buildings resp. influence marker?
Quite true, unfortunately . That's the risk of occupation or conversion, they aren't immediate.
6) In the predetermined setup Carthage is quite squeezed in the southwest corner. Even the 2 resource province east of Carthage may go to Egypt, since he has the Political Leader in the first turn. Thus I think the influence marker and caravan south of Carthage should placed in the 2 resource province instead. With this change every civ controls provinces with 7 resources at the beginning of the game. For Greece it may be better to give him Troas instead of Constantinopolis?!?
Resource count isn't everything. You should keep in mind that this is not a grow-and-conquer game. This is a game about growing without attracting unhealthy attention, and getting to the 4th Wonder / Hero first. If you forget this—and you will on the first two or three games, because the temptation is very strong—then Mare Nostrum will be a 'meh'-kinda title. Once the game 'clicks', you will see that every civ has strong and weak points. I will add that Greece is the hardest nation to play until you learn its 'trick': hide on the isles, and don't be agressive on land.