Even if they themselves shown DNA for being there it wouldn't be a open and shut case. Also it was never pointed out if they had gloves or left DNA behind, they didn't beat her, spit on her, etc. As long as they wore clean cloves that avenue for them might be out, granted if the dust/dirt/etc. for show size and shape could match up and all that but strictly speaking DNA isn't end all be all at this time.
Second thing is that is a personal preference for what to unite against. To me I would rather have something solid, something knowable than just jumping at shadows. You have a firm direction then to go towards and you won't be left flapping in the wind unsure.
As for boring interrogation scene? Eh that is also personal opinion, it might bore some but it can also be used to show not only his competence but how far he is willing to go to his own people that betray. It would stand out in the readers minds what kind of person he truly is along with what this particular Death Note might be able to force upon someone as it is different.
The last thing I would like to mention is this was the third chapter of the story, with the second being a interlude dealing with only Emily Piggot. This last chapter, the third, deals with one person that was in chapter one and from their perspective. We could have seen from him what Kaiser is truly like and all of that.
Now if this was Max's interlude, as it seems to be one chapter per person at the moment, than I would really expect things to go that way. The thing is if his comes up later most of the section that could have been used to show his thoughts, his viciousness and willingness to find out won't be able to be shown. Instead it will probably just be him mad and ordering people about to no effect, then grabbing these people before going there to Emily's house followed by murdering them. The ending of such a chapter would most likely be him wondering off stage wondering what to do now.
To me that would feel kinda empty, but that might just be me.