Hi,
Just wondering if you have got it working? I am having the similar problems, the only way I can see qwt in qt is to manualy copy all files from install dir (/usr/local/qwt-6.1.0) into qt directories (where I gues it finds it) and I can see widgets but when I debug/ run will fail (I actually forgot what the message got too frustrated and got rid of it).
I can not understand why simple plugin install have to be so hard?