Hi, thanks so much for playing and for the feedback! i'll make notes of these. I'm glad the GoldCoin effects stack, I could swear we hadn't fixed that yet :sweating: But good :D I'm not surprised there are bugs with this. The goldcoin upgrade to spawn it is still fairly new and we never build the system for multiples. So it'll all have to get adjusted. This is a great bug report.
For playing with different buffs and stuff, the way I test the game is to Clear Save pretty often. Easier in unity than in the real game, but that is an option if you get tired of your run or want to roll on things quickly. It's in the Settings Menu.
Out of curiosity, what would be helpful for you if we had the option to turn off the achievement/prestige bonuses? I have never even considered having that option.