Personally, I don't think the OP has too many concerns. Laptop, notebook and netbook computers generally are built to take a bit of knocking around. I think the need for a solid state drive is also overstated these days, and particularly with the problems of wearing out that have been mentioned in other threads here; the hard drives on our computers are designed to "park" to avoid damage from vibration.

It's a bit like the issue of carrying eggs. It seems a lot more difficult when someone hasn't done it, than when it's actually being done on a bike. The OP has nice fat tyres, and that is a major plus in damping out the big hits.

As to the heat, check out the temps most laps tops generate, and it's likely that it will be on a par with what can be expected inside a pannier.

The pannier situation does worry me, however, simply because if anything damp or wet is put in the same pannier as an unprotected computer, the water will evaporate and make its way into the electronics. I know people love their Ortleibs, but they do have their weaknesses. A dry bag to put the computer into and seal it off is a simple, cheap solution, along with a neoprene sleeve, and the combination can be carried in the backpack or pannier without fear.

The other issue is the power supply. I don't know what the Macbook Air uses, but we have a transformer, and that needs as much protection as the computer itself, in my estimation.