Lightmap artifacts

Good day. I got this situation.
I have a model (not mine) of a room (3ds max) that I need to put into ue4.
However, the model itself has a messy topology (picture 1), which creates problems when I try to put loops through it and chamfer them.
When still I put a loop that is not straight, I get the result marked with red - soapy and incoherent, and shadow artifacts are still present.
I tried to cut objects into a smaller ones, but got the result marked with blue.
I`d be glad to hear some ideas. I also need some good lessons on creating lightmaps and working in ue4, if you got any (preferably fit for n00bies).