I'm not able to tell you what's wrong, because without code it's hard to find the issue. If you want, please post the part of code, maybe we will be able to help you somehow.
First of all, thanks for the post. GrandNode is pluginable software, it's better to extend its functionality by plugins. To be honest it's also the easiest way to implement changes in it. Is there any reason, why you prefer to do it in source code, instead of plugin creation? GrandNode developer documentation covers topic of plugin creation, because it's the recommended one. I don't know your exact requirements, so I'm not sure on which plugin example you should focus.
It's impossible to create an email account without providing user and password. It's related with MailKit library. You can always submit an issue on MailKit GitHub, maybe authors will be able to make a workaround for you.
We've removed it from the code. I will create the work item on GitHub and we will fix it.
Free accounts on mlab has limits and it can cause the problems. Have you tried to use Mongo installed locally?
Linked products are different than variants used in Grouped Products. In the case of Grouped products, you should assign "Associated products". Tab visible below the "Linked products". If you add at least one associated products, this text will dissapear.
Can you share more details about your environment? It's local or i.e Azure? Linux or Windows? Which version of GrandNode do you try to install? Have you followed our installation guide?
Out of the box it's not possible. Customer can create an order from the whole shopping cart.
Please make an order on our marketplace, I will add the Google External Auth to your order. But please note that previous versions of this plugin are not supported.
Sorry, but I don't understand, maybe I'm missing something. To run application with Plugins, you have to build them before launch. After succesfull build, you will get compiled plugins. Then if you want to change something in the plugin, you will have to open source code, change the code, then again Build the plugin. It's the way how it works.