I think there are a couple of points which are confusing. I take the normal design sail area as around 15 knots real - 17 or so apparent. The Harken calculator gives the force presumably for that sail area in whatever wind speed. Sure if you get gusts to 30 you will be well overpowered, like about 3 x. You would want to have reefed but if you haven't as may happen, various adjustments will occur ie heeling and spilling wind. I doubt the calculator adjusts for that because it is a function of the boat. The boat may also round up, spilling wind. However the basic point would be that you could have difficulty releasing the mainsheet because the loads would be greater.
My point about the traveller is a bit simplified. Setting aside light winds, having the traveller down and the mainsheet tight both flattens the sail, and by decreasing twist can make the leech tighten giving a loss of power in the upper sail by loss of laminar flow. That is fine in a reasonably steady wind. However the other approach is to let the upper part in effect luff, by letting the mainsheet out which is fairly instinctive in a gust. That also lets the boom out which may be temporarily helpful, except that the jib then makes turning upwind harder as the rig is unbalanced. Accordingly you can maintain some drive and balance by moving the traveller up and the mainsheet out, or simply moving the traveller down and the mainsheet in or out depending on whether the response is enough. They interact.
A difficulty in a gust is that the apparent wind moves aft increasing heeling, and maybe requiring you to head up. It may be that you are steering and have to use one hand or your wife has to handle the main. That said you are probably cruising, and want to be comfortable and secure rather than wanting to get into any complexity.