First of all, capable of patience != pouvant de patience which means nothing.
In french, you’ll find posséder de la patience or avoir de la patience or être patient or prendre patience or faire preuve de sang froid.
I think the last one is the best for your meaning of capable of patience.

These don’t exist :
n’en en pouvant plus;
en en ne pouvant plus.
The one is gramatically correct, but never used. en n’en pouvant plus

In general gerund is unused in french. To place negation in your sentences, you’ll have to place it on the previous verb. I couldn’t find a way to use gerund on faire preuve de sang froid. Example : The surgeon is operating with composure. = Le chirurgien opère en faisant preuve de sang froid. To add negative : The surgeon isn’t operating with composure. = Le chirurgien n’opère pas en faisant preuve de sang froid. To add negative with “plus” : The surgeon isn’t operating with composure since his accident. = Le chirurgien n’opère plus en faisant preuve de sang froid depuis son accident.

For your information, “plus” is used for something what had been positive.