All of the manufacturers of spring hanger kits that I have used say not to weld the spring hanger down the side....only across the ends. This will also draw your trailer up.
Regarding the axle/brake thing. Most states require two brake axles over 3500 lbs gvw capacity. The states that require one, like my state, do not conform to fed requirements and if you go across state lines that can create an legal issue...and fall under federal interstate statutes. I am sure no one is going to check, unless it gets involved in a fender bender or you have to go through a weigh station. Here, when you get your trailer inspected by a law enforcement personnel, they are supposed to take a look at such things in order to get a homebuilt trailer ID (vin). But now even, when I build one, and if I tag it, they just hand me a homebuilt id number at the tag office without it.