I wouldn't suggest opening up a keyboard - scissor type keyboards are a pain to work with, and simple enough that there's very little you can do.
Granted that i'm a thinkpad fanboy, i don't think there's very many laptop keyboards that are better. I've also taken these apart, and its simply a little rubber tube which a button pushes down - and these are bonded to a circuit board. I suppose if you could change these tubes (hard) or somehow weaken them (airgun?), you could soften it
Good news is, any standard rubber dome keyboard should be lighter than your thinkpad - just get the cheapest you can. Its not what i'd normally recommend, but in this scenario, its the cheapest option
If you want to do it more scientifically, the thinkpad has a force of between 61 and 67 grams according to geekhack.You can look at other actuation forces there.