The length of his erection does not really matter because anything past 4 inches will certainly contact the cervix. I guess that in this life, anything is possible. So hypothetically, if the string were to contact the space between his skin and where the piercing lifts out, and then it gets looped around during movement, it could theoretically get pulled. It seems, however, that the chance of this would be minimal, and lubrication would certainly decrease the chance of this happening.

I suppose the strings from Mirena "can" get caught around the piercings, but it is doubtful. It really depends on how short the strings are cut. Your medical provider can cut those strings short to ensure that this won't happen, so you should ask to get them trimmed just to be on the safe side.