ColdSteel, let me try to provide some answers (note: not all answers may be correct, some of them are personal opinions based on logical deduction, experience and memory as far as I can recall it. I am not an appointed elder):
1) Number of elders per Kingdom Hall: Variable. First of all it depends on how many congregations share the Hall. Yes, the number of elders in a congregation depends on the number of publishers, and number of groups, and on the spiritual qualifications of the men assigned to oversee the groups (groups may be overseen by ministerial servants, if not enough elders are available). Our congregation has around 100+ publishers, and 8 elders.
2) The extent of their authority: Elders are appointed for both spiritual and administrative leadership in the congregations. The extent of their authority is defined by an array of factors (in the order of importance): direction by the organization (usually letters from the branch office and the circuit overseer), elders' manuals (Sheparding and Flock books), the Bible according to Witness interpretation, locally applicable secular law. One more thing: usually it's not what one elder says, but the body of elders in a congregation. The elders should collaborate in unity.
3) Speak with DF'd individuals? Short answer: Yes. Long answer: While they're the only ones officially allowed to speak (even spirituals things) with disfellowshipped persons, they do it only under certain circumstances.
4) Influence in judicial decisions: in theory—elders should judge things objectively, reasonably and lovingly, and decide the matters according to the Scriptures, without being influenced by personal feelings; actually, things happen sometimes differently. I can't comment on experiences, because all I heard of are rumors and I don't want to comment on them.
5) Is crossing an elder dangerous? Depends on the topic, I mean in what aspect is the elder being crossed. As a person or as an elder? (the latter usually is interpreted as violating Scriptural principles and is being treated according to procedure)
6) Qualifications: see Sheparding book/Flock book, and Paul's pastoral letters (especially 1-2 Timothy, and Titus). That's the short, official, and theoretical answer. In practice, to be able to be appointed as an elder, you have to have been baptized for at least 5 years and have served as a ministerial servant for at least 2 years. The brother desiring to serve as an elder must prove good teaching and preaching skills (reflected by his field service reports), loyalty to the organization (this is more important than loyalty to the Bible according to the charter of certain local corporations used by branches) and willingness to serve the interests of the congregation as a whole. The illustration of the "brown-noser" in 3Mozzies' post may prove 'beneficial' in certain congregations, but that's definitely not the general case. Many brothers who are considered by 'apostates' to be brown-nosers have sincere interests in serving, but that's their way of showing the willingness.
7) How long do they serve: Indefinitely, i.e. as long as their situation allows them, or while they do not engage in certain type of disqualifying conduct.
8) Direct supervisors: The bodies of elders directly report to the circuit overseer.
