The 7 and 9 refer to the heat ranges of the plugs if my memory is correct. I was recently looking at the Bosch web site and one of the pages there gave the meanings of all the letters and numbers in the plug designations. If you Google on "Bosch spark plugs" you'll find this information.
I don't know if the HR7 plugs would cause stalling but the wrong heat range on a plug can certainly result in the plugs getting fouled. What was the condition of the plugs you removed? A plug which runs too cold will not burn off deposits and will eventually foul and misfire. A plug which is too hot can eventually have damage to the ceramic insulator at the center point and can also cause pre-ignition (pinging). Sometimes an engine which has a problem with oil fouling of the plugs can be improved, for a while, by switching to the next hotter range of plugs.
I ran into a similar problem getting the H9DC plugs as you did. My on-line supplier sent me HR9DC and told me that Bosch was recommending them to replace the H9DC plugs. However, when I went to the Bosch site I found a whole list of plugs including the H9DC, H9DC0, and several resistor plugs. The supplier told me that Bosch had told him that they were not making the H9DC plugs anymore! The Champion S12YC are also recommended by Mercedes but I've not read any comments pro or con on these plugs. Most on this forum seem to recommend Bosch. I've used NGK plugs in my Volvos in place of Bosch and have liked them.
If you do some searches on spark plugs in this forum, you'll find that folks are having problems running resistor plugs in engines with ignition systems designed for non-resistor plugs. The plug connectors and also the rotor ( I think) have resistance built in so resistor plugs may add too much resistance to the ignotion system. The purpose of the resistance is to reduce radio interference.
I have the same engine as you do. I finally went to a parts house locally and they carried H9DC0 plugs which I installed and my engine idles so smoothly I can't feel it running.
I doubt that the plugs are causing stalling unless they are fouled very badly. Incorrect or fouled plugs would be more likey to give a rough idle, poor acceleration, and poor economy. Your experience with the cleaner may be an important clue and others may have some ideas on that. In any case, I'd go ahead and install the proper H9DC or H9DC0 plugs.