Do you remember the error code you were getting? It could have been a coincidence.I know it was a ban. I tried running MK7 online without CTGP7 and it would not allow me into the course. After replacing my local friend code file though,
Have you heard of other people getting banned for using CTGP7?
If there was an easy way of accessing foreign eShops, everyone would know about it by now. This is much demand for this feature.why not make NAND backup and then reflash the stock Japanese CTRNAND? From there rip a copy of the Japanese eshop and use that on your 3ds?
If I'm not mistaken, there's some kind of check for an identification code that is unique to each console and that is not easily changed even with CFW. See for instance this guide (linked from https://3ds.hacks.guide/region-changing ).