Whites out of an Erminette project should be dominant white, yes. The Erminette pattern is the same as the Paint pattern in Silkies, one copy of dominant white that allows black flecks to 'leak' through. Those that are solid white instead have two copies of dominant white, so none of the underlying black feathering leaks through in them. That all said, in breeds with White varieties that are based on recessive white such as Ameraucanas, it's not unheard of for recessive white to pop up in other varieties. So while I would assume your White Ameraucana are dominant white in this case, there is a slight possibility that they could be recessive white as well.
As for the single-combed individual, it would depend on who you ask whether they'd call it an EE or an Am, but I personally would consider that one at the very least a cull unless it ended up being the only one with phenomenal type or something. Single combs are not proper for Ameraucanas and breeding that bird only risks perpetuating that trait in your line.
Any of those colors could technically breed together fine as they should all be based on extended black with different diluters. Many say not to breed Blue or Splash with Lavender as it can be difficult to tell the chicks later on down the line that have both lavender and blue genes expressing from the chicks that have only lavender genes expressing, but the rest could all be bred together without issue.
First generation cross of your Whites (assuming they are all dominant white!) with Lavender should just get you Erminette carrying the lavender gene without expressing it, as lavender is recessive. Crossing those chicks back to Lavender would get you a small number of Lavender Erminette, though.
Dominant White to Black would also get you all Erminette, no lavender gene carried this time of course.
Dominant White to Splash would get you all Blue Erminette.
Dominant White with Erminette gets you roughly equal numbers of White and Erminette in the offspring.
Crossing Erminette to any of those colors should result in Erminettes as described above, plus non-Erminettes of the same colors (so crossing Erminette to Lavender would get you Erminettes carrying lavender and Blacks carrying lavender, etc.).
What you should focus on really depends on what your goals are with these birds. If you want to focus on breeding the Erminettes only without the other colors in the mix to worry about, group your dominant Whites and Erminettes together, and you could throw Black in there as well. That grouping should get you all three colors, the ratio of which depends on how many of each variety is present in the pen and which is the rooster, of course. If you want the different kinds of Erminettes I mention above, pair accordingly. If you just want pretty birds and a variety of colors, throw them all together and see what you end up with!
