My preference is to add the bacteria last, preferably into the pad level with injection. The bacteria is unique as it will multiply as long as there is source material. Most bacteria products contain a wide spectrum of bacteria capable of producing different types of enzymes : protease, amylase, lipase, etc. The bacteria cultures is like setting up an enzyme factory as the appropriate bacteria produce the right enzyme in the correct amount to deal with the organic source material. An enzyme is finite and non living which breaks down organic source material and should be extracted at some point to remove the contaminants. Bacteria is living and multiplies producing enzymes to break down organic source material. Bacteria goes one step beyond enzymes as it digests the organic material through the cell wall. Also once the organic material is digested, the bacteria goes dormant forming spores which can be reactivated at a later date. If the spores are at the pad level, then their presence will not as likely to be removed through vacuuming. This is why I like them to remain at the pad level.
Bacteria can be diluted further than the label recommends if time is not an issue. Bacteria will multiply in the presence of organic source material. If you apply it at the point of activity then the bacteria will start at a higher level so that its work will be done faster. Dilute it further then the work will get done, but at a slower pace. Remember that the words enzyme and bacteria are not interchangeable, but are different in the ways described above.