You cannot just correct i for inflation and not correct R_t2 for it at the same time (since you would sell at higher, already inflated prices). In fact, you can't even correct it (W_t1*(1+(t2-t1)*i)) for inflation at all (since your costs are fixed at t1). You buy raw materials at old uninflated prices, and now you suggest we should recalculate their cost at new prices when we sell finished goods (that is i = i0 + p)? That would be an entirely novel idea in accounting. Strictly speaking, you can't even write R_t2 - W_t1*(1+(t2-t1)*i), or that wouldn't be your profit (or benefit, in your speak).
No, you're missing my point.
At t1, if there is an inflation rate p, then interests will be i0 + p, where i0 is "purely economical and independent of inflation" (that is, the market price for "store of value", independent of the currency at hand).
Now, if you buy your stuff at time t1 for a price W_t1, you can consider that you BORROW money at t1 for an amount of W_t1. You will pay back that loan at t2, when you get to sell your product for a price R_t2.
So during the time t2 - t1, you have a loan of magnitude W, on which you will have to pay an interest (t2 - t1) * (i0 + p) * W.
So the extra COST of inflation equals (t2 - t1) * p * W.
If you have deflation, this is a gain (p is negative).
Now, you are right that this doesn't entirely compensate the "gain" due to inflation, of the increase of R.
Indeed, you can say that R_t2 = R_t1 * (1 + (t2 - t1) * p). So R_t2 is bigger (in nominal value) and it "brings you" indeed an extra amount of cash which is (t2 - t1) * p * R_t1.
If your sales price were exactly equal to your cost, that is R_t1 = W_t1, and you would not have created value, then the COST of inflation, W_t1 * p * (t2 - t1) would cancel out ENTIRELY the 'benefit of inflation' R_t1 * p * (t2 - t1), because it is equal.
In the same way for negative p (deflation) of course.
However, if you create value, that is, R_t1 > W_t1, then you do have a small difference: you gain somewhat if there is inflation (on your benefit) and you loose somewhat if there is deflation (on your benefit)... at least in nominal numbers. However, as your gain in nominal numbers, can now buy LESS, that is even corrected for a second time.
But if you make a benefit, you will still make a benefit, whether there is inflation or deflation. Because the interest on the borrowed money to do your production is corrected for it.