Yeah, coastlines are hard. I usually make at least five sketches before I even get close to something I like, and I do it by hand which probably is easier, so I understand your struggle! What I meant by "stitching together smaller segments", was, for example, instead of taking all of Italy, cut it in half lengthwise and use the two pieces in different places.

For the mountains, it's not necessary to know the tectonics, it just helps if you want that extra realism, but placing mountains where it looks good works too Just keep in mind that they form in chains, and islands are mountains with only the peaks above the surface.

By interior lakes, do you mean inland seas? Lakes are created by rivers so are only indirectly dependent on tectonics. Inland seas are like the Mediterranean Sea, with a thin passage connecting it to the ocean.