I've never heard of "lead fusion" on valve faces, and I can find no technical references, or online discussions about it. The generally accepted principle is that the lead compounds in avgas (primarily TEL or Tetra Ethyl Lead) change under the heat and chemical reactions going on during combustion, to form Lead Oxide deposits on combustion chamber and valve surfaces.
I have never seen pure lead deposits in a combustion chamber, because combustion process temperatures are normally high enough to vaporise metallic lead, and because there are a wide range of chemical interactions going on in the combustion process, metallic lead will rapidly form other lead compounds, which are often erroneously referred to as "lead deposits". They are lead compounds, and other chemical compounds, deposits; not metallic lead.
Ethylene Dibromide is added to Avgas to scavenge undesirable deposits of Lead Oxide, and when Ethylene Dibromide reacts with Lead Oxide, it forms Lead Bromide, which produces the light brown/ash-coloured deposits we see in exhaust system components.
It's a little hard to tell from the photo what the irregularities are on the flat part of the valve face. Are they removable deposits, or are they deposit build-ups or disruptions of the valve face metal? The recessed section of the valve face shows distinct metallic erosion and pitting, which is normally caused by contaminants in the fuel combustion process, such as water.
An engine ingesting very moist intake air, takes in a lot of water in vapour form, and erosion and pitting on combustion chamber components is created when the water is heated into steam, and the tiny steam bubbles then implode, eroding the metal. It's essentially cavitation taking place during combustion.
The water and steam in the combustion chamber also cleans off any protective deposits on combustion chamber components, thus exposing the raw metal surface to chemical and cavitation attack.
Acids are also formed during the combustion process and these also adversely affect the metal surfaces exposed to them.
Below is a link to a Shell company article, discussing lead fouling in aviation engines. I believe it's a worthy read to add to ones knowledge bank.
Lead Fouling | Preventing Lead Fouling in Aircraft | Shell Global
WWW.SHELL.COM
This article discusses lead fouling, what causes it and the most efficient way to prevent it in aviation engines. Read the full details here.