The aux fuse [ F22/2] that you replaced was correct. That was the next step in diagnosis.
The problem one runs into with jumping the R15 is that the wire from the low fan relay load terminal to the R15 terminal is too small for the additional current draw. The low fan with the resistor draws only 1/2 the power of high fan w/o resistance... which is why the Factory went with a lighter gauge wire for low fan load circuit. Some guys do this R15 jumper as a modification in temperate climates to gain the additional air flow for condenser. but they rewire that connecting wire to a larger gauge wire.
If that is a plan, the wire that has to be beefed is the one blk/vi connected on R15 [ single wire side] back to K9 relay , terminal #1.
I will also answer a couple of your questions for future reference and to help in your understanding.
The fuse #7 is used to feed the coil side of both relays [ high/low]. This is only the primary side of the relays. If the relays do not CLICK when checking the sensors, this is where one would look . That is why I asked about the clicking. The fuse schematic actualy refers to this fuse as K9 relay fuse, not aux fan fuse. It just power the relays, not the fans. Now , the other part of the relays is the LOAD side ..That is the contacts themselves and they have fuses seperate from all else. That is the aux fan motor fuse you have at the tower.
On your chassis , they combine the two relay in a single case , but they work the same as the dual relays of the 124 chassis 6 cyclinder cars.

Here is your relay . It should have the schematic on the top of the case.

http://catalog.worldpac.com/mercedesshop/sophio/image.jsp?title=Auxiliary%20Fan%20Relay&url=http%3A//img.eautopartscatalog.com/live/P205970192OES.JPG

You seem to have a handle on it now. I do have yor 124.034 schematic up on a screen here for diagnosing this post, so if you PM me an addy, I will send you a jpg if desired.
I would stay with the R15 unit, but I would also do a resistor bridge modification on the coolant temp sensor on the block to lower it's cut-in from 105 to 100C. This gives you the edge when high ambients present themselves. The main guy you want to protect is the engine coolant..It is the lack of a low fan with the a/c on that causes the coolant to jump b/c the condenser is passively heating the radiator and taxing it's heat transfer capacity. Low fan keeps both that and a/c high-side pressure in check.
If you have any other ???s, I will answer them.
You can do that coolant temp in a couple of min and for $2.
RS item/ Archieves.
You can also get a new R15 here from the Forum

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU23G0SK109&year=1992&make=MB&model=400-E-001&category=G&part=Aux.+Fan+Resistor