First of all, I don’t think the Bible really comments on whether slavery is right or wrong, it just simply acknowledges it as a common practice at the time. Remember, in America the morality of slavery wasn’t even really debated until the 1800’s, and both people for and against slavery cherry-picked their arguments from different parts of the Bible (which is why a magesterium is needed, but that’s beside the point). Today, we find even the idea of slavery to be reprehensible. Regardless, it seems as if, at least as far as Israelite men are concerned, that the “slavery” mentioned was intended to be more of an indentured servitude, in which the person was freed from “slavery” on the sabbath year, unless the servant requested, when it was time to be freed, to be made a slave for life. Women and captives, of course, were made slaves for life.
As for the Moabite ban… well, for the most part, it seems to me that it had to do more with the Moabite religion than anything. The Moabite religion included child sacrifice to Molech and ritual prostitution (mentioned when the Israelites were involved with Baal of Peor), both things that were abhorent to both early Israelite religion and later Judaism. As Ruth showed, it was possible (however rare it might be) for an Israelite to marry a Moabite, but only if the Moabite completely renounced all attachments to the Moabite religion and wholeheartedly sought out the Lord. The Israelites were generally discouraged from marrying outside of the Israelite people, primarily due to religious differences (seriously, it’d be hard for a monotheist and a polytheist to understand each other, anyway) - but were practically banned from marrying people from lands that practiced extreme abhorrences in their religious practices.
Just so you know, Paul himself recommended that widows who wished to remarry should marry Christian men. The understanding was that, okay, often first marriages were arranged, and the spouses (especially the women) had little or no say in who they would marry. But widows could choose their spouses, and if they were to choose, it would be better if they married Christian men.