1) Top-ups can be calculated in a few ways, I would suggest that you ask your HR department to confirm what your gross monthly pay will be during Mat Leave and what the lump sum will be.
2) If you take 6 months Mat Leave from Oct, your leave will end in March, If you take some unpaid, you will probably still be back during the same tax year, so you will possibly get one really large paypacket, but the unused tax credits will all be used at that time,
If were leaving in June and due back in Feb, then it would be different, as you might have unused tax credits which therefore would not be used as you would be getting the lump sum in a new tax year.
4) there should be no difference in the total tax paid, unless you take loads of unpaid leave and you end up with unused trax credits.
If you opt to be jointly assessed, you will have more income while on Mat Leave as your husband can use more of your tax credits, but you will be taxed more on the lump sum.