My knowledge of geophysics isn't as good as my knowledge of Geodetics, so I can't give you as much help as with the projection stuff. Of course, you may consider that a good thing Still, this is all just to the best of my knowledge, and I may make some mistakes.
You don't generally get long skinny plates, particularly not with a constricted portion in the middle. Both the continental part of a plate, and plates as a whole, tend to be compact shapes because bits that stick out tend to snap off.
Coastlines generally follow the continental margin fairly closely, but you can have a few deviations to produce "continental" seas (Knowing where your continental margins are would be a big help, continental seas are often great fishing grounds like the Grand Banks). Volcanic arcs are generally produced by subduction of an oceanic plate, so they don't occur far inland and of on a continent, are often mixed with accretion type mountains like the Cascade Volcanoes in the North American Cordillera. The other kind of volcanic arc is produced by a "hot spot" under a moving plate (such as Hawaii) Either way, the volcanoes won't form a dense wall so much as a line of dots.
If you want to place natural barriers, you might consider deserts, barren shield, and other things besides mountains. Mixing your barriers gives you a lot more flexibility. Just a shift in the the local climate or biome can make an effective barrier by simply being different rather than difficult. Even just a river can work, or a lake or inland sea. You also have a lot more flexibility with older mountains than with young ones. The Urals for instance are deep within Eurasia, but are comparatively flat and smooth compared to younger mountains. The Appalachians are another example of a very old range, though they are in a more conventional location.
Islands tend to be hilly or mountainous submerged continent, or oceanic volcanoes. Oceanic volcanoes may be the aforementioned subduction zones (in this case one oceanic plate subducting under another, as with the Aleutian Islands), or Hotspots (Again, Hawaii), Triple Junctions where three plates meet often have particularly large or numerous islands, particularly if the plates are converging. Iceland combines a midocean rift with a hotspot to similar effect.