A plywood subtop is only required for 2cm stone. 3cm stone does not need it.It rests directly on the cabinets. I'm not sure why you think a plywood subtop would interfere with your drawers. It lays on top of the cabinets, not recessed into them.

As far as the support needed for a soapstone sink, it will depend on if you purchased the right sink base for a farm sink or not. If you purchased a farm sink base, all you will need to do is place the sink on the plywood platform and shim until the lip of your sink is at the same height as your cabinet top. If you need to rebuild an existing sink base for a farm sink, then you will need to remove the doors, false drawer fronts, and cut through the horizontal rail between the two to remove it. Then you will build a platform with 2x4s and 3/4" plywood at the correct height needed to get the rim of your sink flush with the cabinet tops. You may need to remove the toekick and reinforce under the contact points of the 2x4s in order to transfer the weight of the sink correctly down to the floor. It will depend on how thick the flooring of your cabinet is and in what condition it may be in. After the sink is installed, you will need to use a cafe rod to hang a curtain over the exposed plumbing below or remake your doors into shorter ones (pretty darn difficult for most cabinetry) and reattach them.

There was no underlayment put down for my 3 cm soapstone counters. It was placed directly on top of the cabinets, shimmed and siliconed in place. My farmhouse sink sits on top of 1x3 supports that run front-back spaced to allow enough room for the plumber to work on the drain and garbage disposal.

Our soapstone was installed right on top of our cabinets with no plywood underlayment. Our undermount stainless sinks rest in part on the cabinet boxes themselves and also on some plywood supports installed where necessary by the installers.
Here's a photo of our first slab laid directly on the cabinet boxes.

Here is a link that might be useful: Soapstone installation

Thank you for your help and the installation photos. I was told the installers do not put in supports so do I have a carpenter put in supports after the sink is in position or before ? Any more comments from Soap stone owners will be appreciated.
Marble is on plywood but soapstone is not, hmmm. Time to do more research.
It LOOKS INCREDIBLY beautiful. both the marble and soapstone(LOVE marble too)
This is a big expenditure for me so I want to make sure I know what I'm doing...

You're most welcome.
We needed the plywood under our marble because of the way the edge was built up. The plywood is supporting the top piece of marble, which would've been floating otherwise. If we'd used a single thick piece of marble, we wouldn't have needed the plywood.
I don't blame you for doing your research. The day of the countertop/sink installation was the single most nerve wracking day of the entire renovation. We had very good installers. At first they weren't going to take our sink away, but in the end they did take the sink in order to template the opening more precisely. I think you are wise to ask questions and who should do what (carpenter/installer).
We love our soapstone countertop. It does scratch and chip a little, but we can live with its evolving finish.

If the sink is a plain stainless one, it's installed with sink clips into the counter itself by the stone fabricators. If the stone fabricators say they don't do this, then you need different stone fabricators. If the sink is a cast iron one, They should also create their own support brackets/cradle. For your own info, look at the Sink Setter mounting brackets. Those are easy to DIY and will support even a heavy cast iron sink full of water if the cabinet below is of good quality.

Thankyou GreenDesigns
The Franke sink does come with clips and fabricator(M.Teixeira) will install them. I am being extra cautious because of the size of the sink(30"x18"). I would like to know if anyone supports these large undermounts from the bottom also.
I have visions of the sink falling down while gallons of water flood my neighbor :( The counter install is probably the most nerve racking...due to the weight and transporting up a staircase. Have you ever heard of mounting the slab on an underlayment of plywood ?

Our sink was installed by M. Tex and it didn't require extra support. They also put our Belvedere soapstone directly on top of our cabinets. They didn't use the clips that came with our sink, they used their own. They installed it after the countertop was installed. We have the Kraus KBU-10 stainless sink...it's D-shaped and 27" I believe.