Which SUT? .... for .3mv/10 ohms MC cartridge into 3mv MM input sensitivity

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In a word, no.  Re-read Almarg's post.Also, you are making this much more difficult than it needs to be.Start with the turns ratio of the SUT, which is another way of stating the boost in voltage it provides.  A 1:10 SUT will boost voltage by 10X, in other words.  The reflected impedance afforded by that or any SUT will be equal to the value of the input resistance of the phono stage (47K in the case of nearly all MM stages) divided by the square of the turns ratio (10^2 = 100).  So, with a 1:10 SUT and a 47K resistor, your cartridge will "see" a 470 ohm nominal impedance.  Since we know your cartridge has a 10 ohm internal resistance, you will be fine with a 1:10 SUT.  The ratio 470/10 = 47, is way higher than the minimum goal of 10X.  There is no problem with having a ratio of these parameters that is >>10. With your cartridge, you can also use a 1:20 SUT; the same 47K ohm input resistor is then seen by the cartridge as 47K/400 = ~117 ohms, which is still more than 10X the internal resistance of your cartridge.
In your final paragraph, you seem to be asking what happens when you parallel 100K with 47K.  I don't know where you got that idea, but maybe someone else talked about REPLACING the 47K ohm input resistor with 100K ohm (not paralleling the two resistors; replacing one with the other).  This is a little trick that one can do if one wants to use a particular SUT with a turns ratio that is not compatible with the internal resistance of a cartridge that has a relatively high internal resistance (e.g., >30 ohms) but a low output voltage.  For example, do the math for using a 1:20 SUT with a cartridge having a 20-ohm internal resistance; you'd want to use a 100K ohm resistor on the phono input side to achieve the desired minimum ratio of cartridge output to phono input resistances. With your cartridge, you don't have to think about doing that.  You can use 1:10 or 1:20, and there is absolutely no reason to wish you could fine tune the turns ratio between those two choices.
I don't want to confuse you further, but the 10X rule is "soft".  You can even get away with ratios a bit lower than 10X but not much lower.  For now, think of it as a hard rule.