I guess if after you clean off the dust and yet the calibration still doesn't work, then you have a faulty touch screen there my friend. Not much more you can do about except to bring it to the shop and hope that they can fix it for you.
I'm not accusing of anyone or any shop but from my experience of buying game consoles and handphones in KL especially in Sungei Wang shopping mall has been a dissapointing one.
especially the one NDSL i bought which has a custom casing. The box is not an original box and they have a ugly manual inside.. and the console turns off by itself whenever it feel like it. I brought it to the shop to fix it until my warranty expired and yet they still can't fix it.
PS. don't buy NDS with the custom casing one.. unless you are sure that it is an official custom casing from nintendo.
Oh and I think the R4 "original" that you got from them is not an "original".The original R4 has stopped producing the flash cart years ago and all the R4 sold here is a clone.. go to GBAtemp to look for a better R4 clone..