I truly do feel for you. Trying to figure things out to make life peaceful for everyone is very stressful. We have all been there, don't get discouraged, you are doing good things.
I was wondering something, you said that Shadow loves the outdoors, but that Marvin would adapt indoors. I know your heart is with Shadow coming inside, but what if you tried Marvin coming indoors first. That would give Shadow his deck back, his safe spot. I don't think he would think you preferred Marvin by bringing him inside, I think he would feel more secure that he didn't have to worry about where Marvin was. If you do have a room where you could put Marvin and then perhaps put a screen between him and Jake, that may help things between them.
Hang in there!!
I was wondering something, you said that Shadow loves the outdoors, but that Marvin would adapt indoors. I know your heart is with Shadow coming inside, but what if you tried Marvin coming indoors first. That would give Shadow his deck back, his safe spot. I don't think he would think you preferred Marvin by bringing him inside, I think he would feel more secure that he didn't have to worry about where Marvin was. If you do have a room where you could put Marvin and then perhaps put a screen between him and Jake, that may help things between them.
Hang in there!!