The use of an all-metal first wall in future magnetic fusion reactors equipped with a divertor may impose severe limitations on the capabilities of optical diagnostics in the main chamber because of the divertor stray light (DSL) produced by reflections of the intense light emitted in the divertor. Here, we introduce a synthetic H-alpha diagnostics to estimate the errors of solutions of the inverse problems aimed at recovering the neutral hydrogen parameters (density and isotope ratio) in the scrape-off layer (SOL) with allowance for (a) strong DSL on the observation chords in the main chamber, (b) substantial deviation of the neutral atom velocity distribution function from a Maxwellian in the SOL, and (c) the data from the direct observation of the divertor. The results of recovering the relative contributions of all three sources to the signal along an observation chord in the main chamber (namely, from the high-field-side and low-field-side SOL sections of the observation chord, and the DSL), together with the isotope ratios in the SOL, are presented for the flattop stage of Q = 10 inductive operation of ITER.