I've Installed them everyday for years. With that being said on our vehicle the module that does security bypass and locks is attached to the data line. I have never personally seen this module cause any interference problems. Especially when writing with hpt. It uses the data line to relay the message for locks or bypass but isn't continously "talking" on the data line.
While something like the aeroforce gauge is always monitoring through data I have seen it cause communication issues (writing w/hpt as well)
And to answer your other question you are writing to the PCM not the BCM. The PCM controls powertrain functions ex. Fans
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