For your first question, regarding shallow overhangs, it is really printer dependent. Some printers are capable of handling overhangs upwards of 60 degrees without issues while others may struggle. Print resolution plays a factor in that as well. And as you mentioned if the depth of the overhang is shallow enough it may not matter at all. To play it safe with averages, most FDM/FFF printers now print between 100-200 micron (.100mm-.200mm) so a .5mm deep or less horizontal overhang of around 45 degrees or so should be capable on just about any printer without support needed. As the designer, it is nice that you are concerned about this, many are not. Quite often, people with the printers are left to figure out if a design is printable or not and how and where to add supports to the model.

For your second question, it isn't entirely critical to model at the resolution of the printer IF your models are more artistic/sculptural/static. Quite often we are used to converting units back and forth between mm and inches as well so that is not a big problem. However, IF your models are more functional/mechanical or have close tolerances or moving parts, then yes, you need to model at scale. Otherwise when the model gets scaled up or down those tolerances are ruined and the model no longer works as designed.

I am just inferring from the information in your post, but it sounds like your design will be more of a historical building type model, in which case you don't really have to worry about designing at any certain resolution. In fact, with models like that, quite often Makers like to print them bigger and smaller anyway. The Eiffel Tower has been printed in every size you can imagine.