Does lur have the kind of negative connotation sly has?
No. Unless you're saying 'lur som en rev' (sly as a fox).
Yes. Yes it does.
Are "flink" and "lur" interchangeable adjectives?
lur = clever
flink = skilled/good at
I don't think you can both pronounce the "r s" and the "s" after the other. You need to choose, in my opinion, because it's a liaison happening here...
'You have a lotion...'
Can lur mean wise