Ball joint made from what? If polycaps ball joint you cant fix it except use something like paper.

If ball joint made from PS and ABS plastic, then you need to add more layer on it. Paint the joint part to fix loose problem sometimes work but I suggest to use putty as layer on ball joint. Sand it if too thick.

Do not use hobby cement, it is not add layer on plastic but instead melting it. I use putty on ball neck joint for my HG Unicorn before. It works better than layer of paint.

Plastic cement can be used, but it is not a first choice because there are much easier ways. The 'melting' process does not dissolve the plastic piece like thinner dissolving paint, otherwise people will not use it to remove seam lines.
I have never bought big boxes in Japan before, after seeing the sad condition of my friend's kit (MG Hi-Nu HWS Clear ) after touching down at KLIA
It's just not worth it and sometimes AA (and generally all airlines) can be very very strict about those and you end up paying extra baggages plus your kit isn't sufficiently protected

Putting up the promo for the warehouse sales.

Yea that's the thing, Tamiya spray cans are synthetic lacquer and based on the information on Tamiya website it will not be affected by acrylic or enamel
I think I must be doing something wrong.
--> you are not doing anything wrong.... even if  you let it cure for 1 week, X20 will still take a bit of the lacquer paint if you wipe too hard and repeatedly.
so make sure your lacquer coat has 2-3 coats... then for extra protection, coat it with a layer of clear lacquer.. that will seal the paint. 
Wah, I did a quick Google search on iwata hp-cp and wow..I really cannot afford that 
--> i have a procon... I have been using the same AB for 5 years.... and works fine.... set me back about 8000 yen. worth every penny.
I am not tooo sure about iwata pricing but invest in 1 good AB is worth it... you are in this hobby for the long haul i assume