Could there be a SD-DPP coalition, or would that be too controversial amongst the members and lacking in minor party support?
Very unlikely, especially after the next election. Of course, there are more and more apperent policy agreements between the two, but in addition to the two things you mention, it would likely be a strategical mistake by the DPP. They would legitimize the Social Democrats completely, and lose their biggest remaining "defining features" compared to them, i.e. opposition to international conventions and the EU. If DPP joins a Liberal-led coalition, you would to a bigger degree be able to see what the DPP contributes with policy-wise. And even that is still a unlikely scenario.
DPP support for a Social Democrat government is slightly less unlikely, but I still find it hard to see happening for some years. If it was to happen, it would probably happen in something like this way: A Social Democrat government minority government comes to power, and insists on carrying out economic policies with the DPP and the left wing and immigration policies with the blue bloc. The Social Liberals despair and decide to pull their support for the government, but the DPP then steps in to secure its existence although perhaps not willing to back it up much in rhetoric. But again, this would mean the Social Liberal trying to convince its voters that it's preferable to team up with the Liberals and get tax cuts and austerity, while ditching the investments in environment, climate and education, that they would actually be able to agree with the Red Bloc on. A hard sell to their cosmopolitan voters, I guess. They probably hope that they can convince the Liberals to go softer on immigration under Kristian Jensen whom the Social Liberals have declared their love to*, but that should be very unlikely unless the Liberals lose their mind.
* The Social Liberals are currently running an absurd poster campaign, that shows very succinctly why everyone else hates them. It is called "Make Denmark Greater". It includes posters with three people who apparently help make Denmark greater: the Social Liberal leader Morten Østergaard, Özlem Cekic who just left SPP due to their right-turn on immigration and Liberal deputy leader and Finance Minister Kristian Jensen. Jensen is apparently a great person, because he is optimistic about the future, pro-EU and has stated he does not believe his culture is under threat. The Social Liberals probably think of themselves as Moderate Heroes, who can make cross-party connections, but in reality this means that SPP hates them for embracing one of their defectors, the Social Democrats hate them for starting a new conversation about the internal troubles in the Red Bloc, and at least parts of the Liberals hate them for giving Jensen the "Kiss of Death" as the Social Liberal embrace makes him toxic in some circles and could help re-ignite the leadership contest in the Liberals if Jensen is too widely seen as "Social Liberal-ish".
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