Like @geekosaur says, you need to read on.
There is a very long standing conflict between Houses Atreides and Harkonnen going back many generations. Early in the novel, Leto sends the Baron a letter reinforcing the state of Kanly between them - a formal state of feud/vendetta, in effect war between the houses. The Baron and Pieter discuss that 'the forms have been obeyed' - i.e. the Baron offered to meet to discuss the conflict between them and the Duke rejected this. The conflict between them and the rejected offer of reconciliation underpin the apparent legality of the raid.
The politics in Dune are very complex as you are probably appreciating. Great Houses each rule one or more planets and it would appear that like nation-states, they do conduct war against each other, with various levels of legal and political restraint. The Landstraad appears to be a weak body, largely in-place to allow the Great Houses to band together to counterbalance the power of the Emperor himself.
The Emperor through his edict requiring Leto to take over Arrakis and his disguised Imperial Sardaukar accompanied the Baron's troops - is clearly involved, but in secret for fear of a co-ordinated response from all the Great Houses. Yes, I am sure there would be fugitives - however the size of the attack was so overwhelming, the financial cost to the Baron so high, that these numbers are probably small. Also, with no evidence of Imperial involvement, any report from someone relatively unimportant will either be denounced as fantasy, perhaps even seen as an attempt to make an excuse for Leto's and his troops failure. Also, a minor character would probably be in fear of their life if they emerged to tell stories.
Legal? Unlikely, but the other Houses have no evidence of this involvement even if they suspect. This is realpolitik.