Agree. it's not convinient. But now situation is so that using native VFB as primary VFB type you can't get denoised picture if you didn't use time/pass/noise stop setup. I reported about porblem with VFB update if Cancel/Esc has been used, so maybe Corona team will take care about denoising using native VFB too.
About denoise amount, Corona didn't ever update native VFB after render stoped. I don't think that Corona team is going to change that, so probably denoise amount spinner will be usable after render stoped only in Corona VFB. Maybe we should contact Rotem with this request...