The short answer is that simply interviewing witnesses is by no means illegal. The question is what was said to the witnesses and the manner in which they were approached.
2/ That said, I'll say that Trump has not been charged with anything, so it's a little bit odd for his team to be conducting an investigation trying to find out what his associates are telling law enforcement. Especially at a time when Trump is loudly insisting on his innocence.
3/ That is, if you know you've done nothing wrong, all you want is for witnesses to tell law enforcement the truth, which in no way requires that you contact them. And indeed, the standard practice here, absent charges, would be to do nothing and assume they're being truthful.
4/ This is especially true when you are the President of the United States of America, and there is both a quality of intimidation and the implicit possibility of a pardon behind any contact you or members of your team have with a witness or someone under criminal investigation.
