I'll get the last question out of the way first: I don't know why the all disappeared from Halloween. The citations in the Oxford English Dictionary have it with the all from the earliest one in 1556 to one in 1616 from Shakespeare (Allhallond-Eue, in Measure for Measure). From the 1700s onward, it's Hallowe'en.
At least until Harry Potter and the Deathly Hallows arrived, the closest word to hallows heard in present-day English was the verb hallow "to make holy", usually in the form of the past participle hallowed. Christians are familiar with it from the first sentence of the Lord's Prayer, in hallowed be thy name; other than that, it occurs most often in hallowed ground(s) or hallowed hall(s), according to the Corpus of Contemporary American English. In fact, hallow and holy come from the same root. In Old English, holy was spelled halig; with the verb-creating suffix ‑ian it produced halgian "to make holy", which underwent several sound changes over the centuries to end up as hallow. The ‑ian suffix is the closest Old English comes to being able to "verb a noun" without changing it. I've translated halgian as "to make holy", since we can't just talk about "holying" something in present-day English, but still, "to holy" or "to holify" gives a better sense of having the meaning of "make holy" encapsulated in a single verb.
So much for hallow the verb; what about hallow the noun? J.K. Rowling used it to mean a sacred object, but that isn't its original meaning. It comes from halga, which as the masculine noun form of the adjective halig meant "holy person". In other words, hallow is a native English word for saint. According to the Oxford English Dictionary, the meaning of the plural hallows was extended to mean relics or temples of the saints, and from there, Rowling went a step further to give it her desired meaning.
Moving on to eve and e'en, both are shortened versions of even, an archaic word for "evening". Even itself might seem to be a clipped version of evening (likewise morn and morning), and cultural historian David Skal even writes, "The word Halloween derives from the Middle English hallowen ... and the progressive contracting of evening to even to e'en." He's wrong here: Evening came later.
Read the rest of Neal's column here.