Almost....

You are 100% correct that a Delta reserve pilot will get the value of a (greenslip/premium/OT) trip paid out on top of his/her monthly guarantee. If reserve guarantee is 78 hours and you pick up a 2-day trip over two of your off days worth 10+30, your pay statement will now reflect 88+30.

You are also entitled to the two off days over which which you volunteered to fly but you cannot pick where they go (if you still have days in the month left). They will simply put them on at the end of your trip. However, if you return from your trip on, say the 30th of Sept, scheduling cannot just put them on for 1-2 October. Those off-days become "payback days" that you can use at a later point of your choosing (provided there is adequate reserve coverage) and, if held long enough, they merely become additional vacation days (not the best use of a payback day but sometimes the reality in a short-staffed category).