1) I would chose 2 10s over a 20 because you have more choices. I thought the clasps would bother me but they really don't and I am VERY OCD about things like that. I also notice that I rarely wear them as a 20, just seem to get more use out of the 10. I also got one of my 10s lengthened for options. I also got an extension made, but that was a couple of years ago and according to several ladies on this thread they do not do it anymore.
2) I think the 10 motif gives you the most bang for your buck and I would suggest saving the money you would put towards a pendant to another 10 in Onyx for example.
3) No
4) No, I personally find it to be too gaudy, but that is my personal opinion. I once saw an older lady at Place Vendôme and she had on EVERY conceivable combination in MOP all at once and it was really overkill in my humble opinion. HTH
If you are matchy matchy or just like to buy in sets (I am guilty of this), I would suggest the earclips, one 10 motif and the bracelet (which really gives you a nice length when combined with the necklace). If you can swing it I would get 2 10s. But variety is nice as well. I *love* my frivole earclips with my vintage alhambra necklace. A good friend recommended this and it really compliments it nicely, so do the perlee hoops. I know I am adding to your list, but I am an addict after all