My concern around your suggestion is that I am summing up the voucher values and putting them into the cart session. If the user does not check out, the total discount value is stored in the session.
Now imagine you have another user who puts in the same voucher code. If the voucher can only be used once, we have the effect of the same voucher being applied twice and both gets the discount.
A workaround I thought to add on to your suggestion is to concatenate the voucher codes in the description of the manual voucher. So, I can run a function to check if the voucher is already redeemed. What do you think of this? Or anyone?
With regards to normal discounts (not voucher based), what do you mean by I should use set manual discount instead of using the database table? Why so?
In any case, since summary discount is not stackable, I should also be concatenating normal discounts with my custom vouchers in a manual discount right?
