Yes, There Are Loopholes..
There are loopholes. What you mention is one: No checks mean no checks. Forget that "the seller may be held responsible", that is not the same deterrent as a NIC check, guns pass through many hands often on the way to a crime, and everyone knows that records and responsibility are diffuse and not easily located in one transaction. It is little in the way of deterrence and does not, in any case, stop the sale.
As well, with the states that have no limit on the number of guns a non-licensed seller ("private person") can sell, the gun show trade can produce numbers that are significant of record-less sales with no check for the buyer being criminal, or the ability to reasonably suspect that there are plans to sell the same guns criminally. It's blind.
This is not the only way illegal guns become illegal but it is a significant contributor according to law enforcement personnel.
Other ways have to do with unscrupulous FFL dealers, guns that are stolen from legal owners or lost by legal owners. The last two are not significant to numbers of illegal guns that have been traced, and it is thought by extrapolation, that they are not generally the main contributor to illegal guns involved in crime.
The flow of guns to criminals is the area of gun restriction/enforcement that make your second question moot: violence in America and it's relationship to other countries and to legal gun owners here.
We who are responsible and legal with guns, are not the ones committing the crimes, yet because so much violent crime does involve handguns it is an easy transference to blame the instrument (the gun) owners, rather than the person committing the crime. But until the flow of guns to criminals is stemmed, this will be the future case too.
By taking a stance that any restrictions on guns or gun sales are on attack on the 2nd Amendment, many on the "Gun-right" and the NRA have stood in the way of reasonable restrictions being enacted throughout many states or federally. Thus, the good of their support of an individual right is mixed by inclusion of an "either-or" stance on all questions of gun topics.
The reason why Heller is of real value is that it will eventually align the laws generally with what the vast majority of the American people now believe (70% from recent polls): an individual has a right to own and use guns, but that ownership and use should be restricted, as are all other rights. Legal test-cases of these restrictions will abound and eventually define what is essential to 2nd A. protection, and what is permissible; these decisions will eventually lead to a more uniform, rational gun policy throughout the states. It is now overly-restrictive in some locales, and lacking in safeguards in others - while still others have a rationale mix of freedom with responsibility.
In other words, both extremes will eventually lose and the vast, reasoned middle will win.
[If you want to study the topics above more, I would suggest staying with sources other than the "far left" (some anti-gun groups) or the right (this forum is not objective on these subjects for example, - though Firing Line is very good in other ways) - or the NRA, etc. Pick from general library holdings or internet search engine findings, various studies of these topics, academic, or think-tank, also include law-enforcement and Justice Dept, FBI etc., as well as published records, historical or otherwise. A broad read, will give you less of the religiosity that infuses the extremes and the obsessiveness of a white/black, believer/sinner mentality. You want facts and rational analysis so you can form your own intelligent assessment.]
Lastly, the point that America has more violence than other countries is generally true, for a complex of reasons. But fewer guns in the hands of criminals - though that will take many years - is now possible, and will help to make the violence from criminal gun use less of a contributor to over all violent crime levels. (All violent crime is, by the way, decreasing - and is now as low as levels not seen since the early 60s. And the broad measures of crime over many, many years extending back to the 19th Cent. always move in a marked downward direction.)
