Hi tsmall-- you raise some good questions. My take on them:
1. Like many astrologers I sometimes think of planets in a horoscope as members of a committee, or members of an extended family. They don't always get along. Maybe this is similar to observing that most people experience inner conflicts or competing wishes. As the Jewish saying goes, "With one tuchus you can't dance at two weddings." In a more choice-oriented astrology, people can work to empower and optimize both planets, however.
So let's suppose for the sake of argument that someone has Venus in Aries and the moon in Libra in opposition (to use your example). This looks like rough sledding, yet the two planets are definitely communicating. Maybe a good use of this combo would be for the person to recognize a Venusian commonality with both placements, and to enter into relationships that are committed yet that grant both individuals a fair bit of space. Libra likes equality (or is at least receptive to the idea) and is able to rationalize things (as an air sign.) Maybe the person has independent and somewhat daring (Aries) artistic tastes (Venus, Libra) which she can express in her home decorating (Venus, moon.)
I have a fair number of oppositions in my chart and have found this kind of deliberate strategy to be really helpful. Mars is one of the easier ones because it can be converted to an athletic activity; and even Saturn isn't too hard because it keeps an eye out for self-discipline and caution.
2. Accidental house cusp rulers are the planets domiciled in the sign on the house cusp. I usually look at both traditional and modern rulers. A good book on this is Karen Hamaker-Zondag, The House Connection. "The house over which a planet rules serves the purposes of the house in which that planet stands." So, for example, if someone has Virgo on the cusp of the 10th house of career and Mercury (rules trade) is in the 4th house of "home" perhaps this person would make a good real estate agent.
I usually use Placidus houses (unless it's a guesstimated birth time or a high-latitude birth,) so you oftentimes get a house cusp sign that is different than the sign of planets in that house. I find Placidus gives a chart a lot more sensitivity than whole-signs, because you can always look at a planet's "natural" sign ruler if you want to. Also, with an unequal house system you can get intercepted signs, which I think have real interpretive value.
But just to bolster this technique.... sometimes people wonder why some Aquarians seem more Saturnine, and some more Uranian. Well, some have Saturn (Capricorn on the cusp) as their sun's house ruler; and some have Uranus.
This method is really helpful for interpreting untenanted houses. If someone has no second house planets, for example, look at the planet ruling the sign on the second house cusp. What house is it in and what aspects does it make?
Don't bite my head off, whole-signs advocates!!! Your milage may vary.
3. I don't follow the belief adopted by some modern astrologers of "natural" house rulers according to their rank-order around the horoscope. [Moon rules the 4th house because Cancer is the 4th sign, and so on.] Apparently Lilly proposed this correspondence for medical astrology, and then C.E.O. Carter and other mid-20th century astrologers picked it up.
Some planet-sign-house combinations work fairly well (likeJupiter, Sagittarius, and the 9th) but others seem very badly matched, such as Mars- Aries-first house, and Venus-Taurus-second house. In the first instance, the first house is your outward personality and body; and it won't seem very "martial" unless you happen to have Aries rising or Mars in the first. The second house is your money house, yet Mercury rules money and trade, not Venus (modern misunderstandings, notwithstanding!)
4. Since I don't work with a lot of the info-bytes of traditional astrology, I can't say whether they explain why someone is unhappy about her love-life, or what-have-you.
But oftentimes you find difficult planetary aspects involved; either in the 7th house or with the ruler of the 7th in the case of the lovelorn. Or sometimes she's got moon-Neptune in the 7th, which can be soooo idealistic.
A tough combo with the 40-and-still-single example is Cancer rising, with Capricorn on the DC. Cancer wears her heart on her sleeve, and closes up within her shell if her feelings have been hurt too often. Saturn ruling the DC can indicate frustration and disappointment-- or simply a marriage late in life.
Maybe I don't use the negatives of traditional astrology because my sun is in Aquarius. Once I learned, Popeye style, that "I yam what I yam," it hasn't bothered me in the slightest that my sun is supposedly in its detriment. I mean, are we saying that something like 600 million Aquarians have a problem with their sense of identity simply because they were born in late January through mid-February?
My Jupiter is supposedly in its fall in Capricorn (4th house), yet I love living surrounded by old (Saturn) books and furniture, studying family history (Capricorn, 4th house), and living in the mountains on a small rural acreage (Capricorn rules mountains and farming). Is this supposed to be a problem?
5. I do think that you can use planets and angles to learn something about other people and events in the native's life: they are not 100% interior. Mom and Dad, for starters. A biggie is a planet opposite the sun, which will often be experienced as an 'enemy" opposed to the native's interests and who seemingly embodies the other planet's characteristics.
But the more self-aware and responsible people are, I think the less any negative exteriorizing is likely to happen. We actually are not Mom or Dad, for example, but we are our experiences of them.