Hello Nuch, I'm sorry about your mother.
You pose an interesting question. I'm from a fALS family. I don't know that FTD increases the chance of inheritance of ALS. I think I remember reading that a genetic marker was found in FTD patients, however not 100% sure.
Your understanding of the figures sounds correct to me. Bear in mind that of the 5- 10% who are fALS, only about 20% of those will have a genetic marker that can be tested. Our family is confirmed by medical history. My mother's grandmother died from what was called at that time "creeping paralysis". My mother was predeceased by two brothers. Mother died in '04. Mother's youngest brother is now in hospice care.
As for testing, it has splintered my family. Not finding a known marker does not mean you are not predisposed, just as with my uncle. If your mother had been tested and a marker found, then you not finding that marker would offer greater reassurance.
Genetic counseling would be required before you could be tested. There are many things to consider. Without a relative with a positive genetic marker and without any other family members with ALS, I expect your counselor would not advise going forward.
Some of our family have choose to adopt. We now have four wonderful children adopted by family members. Others have choose to have children. You may find that genetic counseling will clear your mind and decide to do the same.
If I can answer any questions, feel free to message me on my page. If I can find that study about the marker for FTD, I will post it here.