Hi helpmattress,
As you mentioned a mattress protector is always a good idea with any mattress to protect your mattress from stains and the body fluids, skin cells, oils that we release each night, to protect against spills and accidents, and to keep your sleeping surface in a clean and hygienic condition. It will also protect your warranty because mattress warranties are usually voided with any type of stain on a mattress. There is more about the pros and cons of different types of mattress protectors in post #89 here that can help you decide which type would be best for you.
It sounds like you’ve already removed the stain but there is some good information about cleaning a mattress and spot cleaning any stains here and here and here that may be helpful.
It’s also very important to make sure that the mattress is completely dry before you put your sheets and bedding back on the mattress (see post #2 here).
I don’t know which of the Ikea mattresses you have but moisture and humidity can speed up the softening and breakdown of polyfoam and memory foam. I don’t know if that’s the reason that your mattress is sagging though and and there are also other reasons that could be the cause as well (including low quality/density foam materials that don’t last very long).
It’s very difficult to “fix” a mattress that has softened or is sagging if the problem is in the mattress materials themselves because the most effective solution would be to remove and replace the layers and components that are the cause of the sagging or to replace the mattress itself. Having said that, there are some suggestions in post #4 here that may be helpful … at least as a partial or temporary solution.
Phoenix