All the examples you gave are grammatically correct. I agree that the use of the past form makes the request more formal and/or hypothetical (the whole if- sentence is a second conditional). "Would you mind if I ask you to help?" sounds strange but the others sound good. The reason this one sounds strange is because the natural context would usually be hypothetical. So, "would you mind if I asked..." would express this idea correctly. Technically, [if + present, would + base verb] is a mixed conditional. If I were being a grammatical purist, I could argue that it's not a good or necessary form. In my opinion, you will be fine if you choose these options: a) Present situation. Neutral/only a bit formal. "Would you mind + -ing?" b) Present situation. More formal. "Would you mind if I + present simple?" c) Hypothetical situation or very formal present situation. "Would you mind if I + past simple?"