hi, first - pcmanfm-qt must be a part of lxtqt-core - lxqt-core and lxqt are the meta-packages to install lxqt - beside the package desktop-task-lxqt. As all metapackages and task they can be happily removed at wish.
Hm - not to be unkind, but i don't think that we (the LXQt Project and Debian LXQt Packaging Team) will remove pcmanfm-qt from lxqt-core nor make it a recommend because it is really a LXQt core package.
the icon thing is more complicated - this should be a i3 setting, we could figure it out in IRC