Cute chicks! It is a little too early to determine sex yet. The one pictured above is most likely a mottled "Isbar". It will probably be very pretty, but should not be used in a purebred breeding program.
I see no problem with project colors as long as you keep them separate from a purebred breeding program and clearly disclose them as projects to anyone that gets offspring from you. I am working on a project color myself. What color do you want to work on?
That depends on who you listen to. For me, I wait at least 30 days. If I am able, I will actually separate the hens from the cocks and then check all eggs for fertility until they are not fertile for at least a week, then put them back together.
Very cute! Your center chick does not look like splash to me. You should be seeing some blue coming in on the feathers by now. If you do not see the blue color very soon, I would question whether or not that is an Isbar. Here is a picture of one of my week old splash chicks:
Congratulations on your eggs! As long as the inside of the shell is blue, you are good to go. You may need to remove both of the white membranes to see the blue.
He looks very handsome! I would like to see more green in the legs, but that can be fixed by pairing him with hens that have green or slate/yellow legs. One of the most important things, though certainly not the only thing, to be concerned with in a cock is that he carries the blue egg gene, preferably 2 copies. To test for this, put a white or brown egg laying hen in with him. Hatch the eggs and grow up at least 10 of the pullets to laying age. If all of them lay blue or green eggs, then he is very likely to be homozygous for the blue egg gene (has 2 copies). If some lay blue/green and some lay brown, then he is heterozygous (carries only 1 copy). If none of them lay blue/green, then he most likely does not carry the blue egg gene.