...And I'm sure that slavery is in the Bible. There are even instructions on how to treat one’s slaves. Slavery is not legal and even the most devout can’t own any human beings...
Yep. Conflicting signals. Had believers erred on the side of freedom, Europe and America would not have enslaved many millions of Africans, and we could have avoided many generations of trauma and suffering by those lost souls, and their descendants, and our own Civil War would never have happened, and we would not have lost 600,000 of our own, not to mention the vast and larger numbers of wounded, and all of those destroyed lives, connected to the fallen and maimed.
...Mutilations for punishments are all over the place in the Bible for various crimes including theft and being a prostitute...
Yep. Most commonly by people who do not recognize the simple fact that the teachings of Jesus of Nazareth in the New Testament were designed to supersede those of the Old, with the Old merely there for historical and philosophical background and as supplement for topical matter not covered explicitly or implicitly in the New.
...Christians in some places still practice them...
Possibly. And, if true, only in more backwards areas, where the mindset of the population is buried far in the past - pre-industrial, at the very least, and, usually, pre- Reformation, or extra -Reformation.
...In the U.S. the Constitution prohibits such things; they are illegal...
Thank God. Although this is also true for most places in The West which are dominated by a population largely Christian in confession.
...Usury is condemned as an abomination in the Bible. Are Christians being threatened by the interest rates being charged poor people?...
Yep. Christians are called upon to shun such practices. That doesn't stop the more ignorant amongst Believers and Unbelievers from engaging in the practice.
...The Puritans stoned both gays and adulterers, then later just the gays. Now Christians don't stone either...
Insofar as my own poor and amateurish understanding of such things goes, Jesus of Nazareth saved a Sinner from stoning, and served-up a lesson in shunning Sin while forgiving and protecting the Sinner. Psychotic do-it-yourself post-Reformation delusional Christian-wannabes like the Puritans allowed Old Testament examples to take precedence over the loving and forgiving and protective lessons taught by their nominal Teacher and Master and Lord.
...How do your Christian ideals cope in such an ever changing landscape?
Historically, intelligent and sincere and compassionate People of Faith generally have little difficulty with distinguishing Righteousness from Wickedness in any era, despite the trend or fashion or mood or fickleness or momentum of the times.