Does it reorder? I would expect these two to be equivalent

let x = f (g 1) (h ()) in

and

let f2 = f (g 1) in
let x = f (h ()) in

and in the second case the order is clearly g before h.

It depends on the order of evaluation. I forgot to specify that I was taking right to left here (the order of evaluation is unspecified in Ocaml, if I remember correctly).

If the order is indeed to evaluate the last argument first then

let t1 = h () in
let t2 = g 1 in
let x = f t2 t1 in
let y = t2 in
x + y

It's not hard to preserve the order whatever it may be.

Yes, it is what I meant by "monadic form". To sum up, I'd say exceptions have some interesting properties, but are quite far from pure function as far as optimisation opportunities are concerned.