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.

My preferred method would be to juice the mint in one of those juicers meant for wheat grass which can get a ton of moisture even from a dryish plant, then use this juice pure or dissolved in sugar.

The second thing you can try is maceration. This is usually done with fruit, and means you cut it up and mix with sugar, then leave to stay for several days. With fruit, lots of juice flows out, but with mint, I'd suggest simply adding a little water. It won't split the cream if the amount is small enough. You'll get an even better extraction if you add a bit of vodka. Make sure that you use enough sugar to not have the stuff go moldy or ferment.

Logophobe's idea for infusing the cream also sounds good. David Lebowski has several herb ice creams which involve transferring the flavor to cream, then discarding the plant matter. He calls for boiling the herbs in the cream, and this works very well - almost too well for me, the sage ice cream turned out to be an acquired taste. The taste changes during cooking, but if that's OK for you, cooking up a sugar syrup with mint, or making a strong mint tisane and then cooking a sugar syrup with it, may turn out to be the easiest way to prepare that. The advantage is that sugar syrup is boiled to supersaturation, so you get no free water afterwards to interfere with your cream whipping.