I would not risk using letro at this point.
The puffiness is from the AAS, not necessarily from gyno. It will almost certainly go down after the cycle is over.
The lump, a lot of people have small lumps. Nothing to worry about IMO.
I think letro at this point is too risky. It could negatively affect your recovery or at least complicate something that doesnt need to be.
In the future, you know to use a little more arimidex per that amount of aromatizeable AAS.
Dont use the letro, taper off the AI as planned, and begin the nolvadex as planned. Your issue should not get worse. This is what I would do if it were I facing this situation. The ultimate decision is yours, obviously.