A New Testament Question: Ephesians 4:1-16

Next week's Epistle reading is this:

I therefore, the prisoner in the Lord, beg you to lead a life worthy of the calling to which you have been called, with all humility and gentleness, with patience, bearing with one another in love, making every effort to maintain the unity of the Spirit in the bond of peace. There is one body and one Spirit, just as you were called to the one hope of your calling, one Lord, one faith, one baptism, one God and Father of all, who is above all and through all and in all. But each of us was given grace according to the measure of Christ's gift. Therefore it is said,

"When he ascended on high he made captivity itself a captive;
he gave gifts to his people."

(When it says, "He ascended," what does it mean but that he had also descended into the lower parts of the earth? He who descended is the same one who ascended far above all the heavens, so that he might fill all things.) The gifts he gave were that some would be apostles, some prophets, some evangelists, some pastors and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until all of us come to the unity of the faith and of the knowledge of the Son of God, to maturity, to the measure of the full stature of Christ. We must no longer be children, tossed to and fro and blown about by every wind of doctrine, by people's trickery, by their craftiness in deceitful scheming. But speaking the truth in love, we must grow up in every way into him who is the head, into Christ, from whom the whole body, joined and knit together by every ligament with which it is equipped, as each part is working properly, promotes the body's growth in building itself up in love.

I mentioned to a priest of my acquaintance that I thought Paul really made some bad arguments at times - well, I think he does, although the priest pointed out that it could be the culture gap, and I acknowledge that that might be right - and cited this section above as an example:

(When it says, "He ascended," what does it mean but that he had also descended into the lower parts of the earth? He who descended is the same one who ascended far above all the heavens, so that he might fill all things.)

My point was that this is, to me, a typical Pauline stretch; that "He ascended" does not necessarily imply that "He had also descended" - depending, of course, on who you are talking about. It also has nothing whatsoever to do with "the lower parts of the earth," that I can see - and where on earth does it say, or even imply, that "he ascended far above all the heavens," for goodness' sake? The passage being cited here is Psalm 68:18 - which, it seems to be generally thought, is a reference to Moses:

When you ascended on high,
you led captives in your train;
you received gifts from men,
even from the rebellious—
that you, O LORD God, might dwell there.

Another problem, of course, is the "giving" vs. the "receiving" of gifts. Which is it?

Paul is obviously talking about Christ above (although I'm still not sure what "the lower parts of the earth" have to do with it). I thought Paul was - once again - going way too far, but my priest friend said this: "That shouldn't even be in there! That passage is very difficult." (Or something like that - meaning, I gathered, that it would be better to leave the little aside out of the lectionary reading, because of inherent and/or intractable problems with the translation or something.)

And I've been reading a couple of commentaries on this section, and indeed there do seem to be some difficulties with it, even with the translations of words. Does anybody know what these happen to be, in case my Google Book commentaries suddenly cut out on me? Or don't include something important?

Thanks!