As a pretty experienced Scotland driver, Google times are accurate IF you drive at the posted speed limit (sometimes a little difficult) AND you don't run into coo, sheep, tractors, or other tourists. I add 25% to Google times for planning, and we usually manage to stay on schedule. Skye can be worse than the "mainland".
I don't think you're spending enough time on Skye, or Edinburgh, for that matter. Traversing the entire country for a one week trip is sort of aggressive. You'll get to Pitlochry/Glencoe in the afternoon, and have to leave early to get any time on Skye, so you won't see much of either. Depending on resolving the car problem, you may have to leave Portree pretty early, as well if you have to get there by closing. The passengers will at least enjoy the views.
Don't forget all those routes from town to town are littered with castles, abbeys, scenic lochs, waterfalls, and cafes.
There's also a Bank Holiday the first weekend in May, so expect some traffic if that overlaps your dates. We were headed from Glasgow to Oban and we ran into a massive traffic jam at Loch Lomond that luckily was in the other direction. Literally miles of stop and go.
Sorry for what seems mostly negative feedback.
