What type of lesions might not show up on an mri or ultrasound? after two ultrasounds, a non-contrast mri, u/s vein mapping/doppler - not even edema is showing up on the images, and there are numerous palpable arm lumps (no redness, no heat)?

I suggest that you see a doctor in person. I am not sure how to address your question based on the available details

Firstly, if a lump isn't stone hard and hasn't increased in size over the past weeks, it's a good bet that it's benign. If the density of the lump is similar to the surrounding tissues it may not show up on ultrasound . I know that lipomas, benign fatty growths have not shown up in MRI's done with out contrast. You may have such lipomas. A radiologist is better qualified to answer your question.

The supposition of y: The supposition of your question is lacking details. There are many people that feel that the internet is a substitute for medical objective evidence and feel that asking for details is are not helping them. We live in a day when asking for truth and information is a violation of what should be done. So sad.