About differences TDR and VNA TDR https://incompliancemag.com/time-domain-measurement-tdr-or-vna/ https://coppermountaintech.com/what-is-the-difference-between-tdr-and-vna-time-domain-measurement/ Als

By default at startup always load 0 calibration slot But if select config->expert settings->remember state (and save config, config->save) NanoVNA store several settings in backup registers and use it

Exist USB isolators on ADUM3160 try search it https://aliexpress.ru/item/1005002959825296.html?sku_id=12000022964317764&spm=a2g2w.productlist.search_results.0.48497f1aAGtFNW

No, measure module in NanoVNA not provide any API on console, just display You can read measured data and calculate on CPU side