How to make mint syrup with minimum water?
Well - not exactly. The reason that this technique works with lemons is that they actually do contain quite a bit of water. It's not so much "water-free" as it is using the residual water from the rinds and un-squeezed pips. Mint contains a lot less water by weight, so if you tried it in similar portions you'd wind up not with mint syrup, but with slightly-damp mint-flavored sugar.

However, that might actually work pretty well for your intended purpose. The flavorful oils in mint will dissolve very well in sugar or in something with a lot of fats, like cream. You could try gently muddling fresh mint in a small amount of sugar and letting it sit until the mint looks ragged, dissolve the mixture in cream, filter out any little bits of mint, and then using that to make your whipped cream.

You could also try breaking up the mint a bit and letting it sit in your cream for a while to extract the essential oils, but my suspicion is that you won't get quite as intense an extraction that way.

The other option that bears mentioning is using an essential oil or extract to flavor your cream or cream cheese instead.