Hello,
First understand that when you set the exchange rate between two currencies in the plugin, this only affects the display price in equivalent fiat currency (USD) and also it affects the prices in the WooCommerce extension.
To set the exchange rate manually to a fixed value, you must now introduce a filter hook in PHP, like so: https://gist.github.com/alex-georgiou/492196184f206002c864225180ca8fbb
Please let me know if you have any questions about how to modify the example code and activate it in your WordPress setup.
In the upcoming wallets6 release it will be possible to set exchange rates via the admin interface.
Unfortunately there is no possibility of controlling which currencies can purchase which products.
with regards
P.S. I have moved your question here as it was not related to the other question.