Okay, I see more of the problem now (not just queiries). You are suffering from what nearly ALL software development project go through. You are correct, they (developers) should be looking left and right and using better judgement. But that is true with ALL employees in every position -- some give a shit, others don't and short cut.
What you are describing is the need for Quality Assurance (or software testing). I'm currently us a Ukraine Ruby on Rails developer (I'm on East Coast US) and I run into same frustrations (even when using multiple developers). The password issue is classic (lots of sites suffer from this). Want to see something stupid -- go to most any hotel site (like Hilton) and select room for week of Christmas -- you'll get some stupid "ERROR, ROOM not AVAIL" message at top of screen. I'm thinking, geez, this is Hilton, can't they hire better programmers.
So what to do? You need someone to do software testing (I do my own, it's a pain). Maybe you are the best one to "own the product/User experience". Maybe you can hire someone -- really anyone who is NOT a developer can test to make sure your site works in a manner that will satisfy users. Maybe give one of your clients a discount for pointing out flaws. Be creative!
And constantly send emails back to your developer to say "guys, fix this, fix that" and say "come on at least take SOME time to look at this from the User's perspective (I bet they complain about same password error when they try to buy something online from other site).
Then, REWARD those that do look Left & Right. Rewards/incentives always better than punishment -- people love recognition.
Of course, firing the slacker in the bunch is always good idea too. I had team of 20 developer at my last startup and the good ones were always glad to see the slackers go.
Good luck, this isn't easy stuff.