How can I use a custom welcomepage (Home.aspx) on every subsite I create? When I select the Welcomepage in Site Settings I get an Welcome page invalid error.
Mostly the navigation, left/top and content on MasterPage are the things you want remain same on the sub-sites as well.. But the content of the homepage should differ from that of parent site, it should show the details related to your sub-site!
So if you want your parent site MasterPage to be used by your sub-sites as well, you can turn on the Publishing Infrastructure Feature [Turn on from Site Collection Features and then Site Features]..
After turning on this feature -> In Site Settings -> Under Look and Feel -> Master Page : This will give you an option to use the same MasterPage from parent site for your sub-sites, below is an image:
You need to check Reset all subsites to inherit this master page setting
I hope this helps