For established long distance routes, you can find them on Scotland's Great Trails. These are waymarked long-distance routes, that you may be able to get special maps covering the route. There's also a slightly better overview map at WalkHighlands. Most of these long-distance routes however delibeatley avoid unnecissarily summiting however, which conflicts with some of your desires.
The Southern Upland Way, The Forth and Clyde and Union Canal Towpaths (although this is both short and lowland, almost certainly not what you want), The Great Glen way all appear to go coast to coast. Combining the East Highland Way with some other routes (Speyside way etc) would also seem to provide a coast-to-coast option.
However, if you are after a more isolated, back country route, then you are unlikley to find it on any of these waymarked long distance paths. Instead, I recommend you sit down with a lot of Ordnance Survey maps (you can get the paper versions, use Bing maps, or a software tool like Memory Map) and figure out your own route using public footpaths and right to roam. You may find online blogs of people who have done it before you.