Dismantle basement window and seal it with concrete

Wouldn't formwork blocks made of concrete actually be exactly what lies between both worlds? You could pour concrete in one go without having to worry about the formwork boards. Of course, except for the last piece. How do you see the issue of efflorescence?

If I could just find those strange holders he describes and give the stones support in the reveal

Either bend flat anchors at a 90-degree angle and plug them into the reveal or drill holes and fix plug anchors with fast-setting concrete (from the cartridge) in them. The ends are laid into the new joints and mortared in. Efflorescence only occurs with moisture, or when it dries. And if you absolutely want to reinvent the wheel, use formwork blocks. But it’s just more effort and brings absolutely nothing.