Modify a Teams tab solution from GitHub to match the customer requirments

I am working on a new project and the customer have already installed and used this Teams tab from GitHub @ https://github.com/OfficeDev/microsoft-teams-emergency-operations-center

Now they want to do some modifications to this solution, such as :-

1. Change some titles for the fields
2. currently when an item get created it provision some channels on teams, so they want to modify the name of those channels and provision more channels.

So my first question is about how i need to do those changes,, are those steps valid:-

1. Clone the code from the above github link
2. modify the code accordingly
3. Deploy and publish the solution to get a new .sppkg solution and re-upload it to the sharepoint app

are those valid steps to do the modifications? i built a lot of SPFx solutions, but never modify existing one on GitHub.

Second Question, when the customer install this solution for the first time, a new site and lists were provisioned... so if i replace the old .sppkg file with the new one, will this re-provision the site and the lists? or this will only happen when we installed this solution for the first time only?

Third question, now if i do those modifications inside our local version after cloning the solution, and Microsoft releases a new version of this app inside GitHub, and we want to have this new version, then our local modification done previously will get lost? am i correct? so can we for example post those changes to GitHub as well, so they will be part of the new releases done by Microsoft ?

Thanks