Please review the Q&A Usage Guidelines before posting, specifically point 4, "Make a separate post for each question." This is especially pertinent when asking unrelated questions, such as these, which can make it difficult for other teams to utilize the Q&A as a source of information.
Since fishing line can be single stranded, is 1/8" metal wire legal as well.
Taking it one step further - is coated or jacketed wire rope legal?
Generally, yes, these would be legal, if being used as rope / string.
It is impossible to provide a blanket list of all possible forms of "string / rope" that exist. G3 should be applied when attempting to use "non-traditional" materials for rope/string; ultimately, it will be at the inspector / Head Referee's discretion whether to rule a given component as a legal use of rope/string.
The most important "thought experiment" we would recommend would be to replace the material in question with braided nylon rope of a similar thickness. If the Robot / mechanism still maintained a similar function as before, then it is more likely to be ruled legal by an inspector.
An extreme example could be taking a 1/4" piece of any material and call it single strand rope.
This is an incorrect assumption. It is extremely unlikely that any inspector or Head Referee would consider a 1/4" wooden dowel to be "string".
Delrin is included in the permitted custom plastics in the game manual, but there are varieties of Delrin that contain additives that greatly enhance performance. I assume these are not legal?
Most of the legal plastic types listed in R10 have various derivatives that are still generally referred to by their "parent" material name. We are not going to provide a comprehensive list of every variant of acetal, polycarbonate, etc beyond what is already listed in R10.
The intent of limiting a Team's choices to these specific plastic options is to limit the risk of shattering plastic, such as acrylic, being used on a Robot. Therefore, the difference between "delrin" and "delrin with an additive" is generally irrelevant in the context of R10, as long as the latter is still nominally referred to as "delrin".
If there is a specific material that you would like to be clarified, please provide a link to it in another Q&A and we would be happy to provide more specific guidance.