It will be helpful to give the data,and the wanted final result.
But if the only problem is that paht in $addFields , you can fix it.
You need to use arrayElemAt
.fieldName is ok
.index doesnt work in aggregation (use arrayElemAt)
You can do it by hand using temporal variables.
Or you can make a driver function that generates that MQL code.
My generator function for your path gave
{"$addFields" {"test" {"$let" {"vars" {"v0" {"$let" {"vars" {"v0" {"$arrayElemAt" [{"$let" {"vars" {"v0" {"$arrayElemAt" ["$RulesetInfluencedAttributes" 0]}}, "in" "$$v0.LexisAttributes"}} 0]}}, "in" "$$v0.myattributes"}}}, "in" "$$v0.myvalue"}}}}
By hand its a bit smaller
{"$addFields" {"test" {"$let" {"vars" {"v0" {"$arrayElemAt" ["$RulesetInfluencedAttributes" 0]}}, "in" {"$let" {"vars" {"v1" {"$arrayElemAt" ["$$v0.LexisAttributes" 0]}}, "in" "$$v1.myattributes.myvalue"}}}}}}