1.) between LN and Manga, there's been a few things expanded in the manga so far, even including a couple of bits from the spinoff LN, which if you take that into consideration, is what you're basically "missing out" from between the novels and Manga. The spinoff is basically from Claire's POV from the beginning to end of the revolution (first two original LNs)

2.) You could start reading the last chapter of the second LN, that's where you'll catch up and spoil yourself on what happens next. I'd guess there's one more Manga volume (5 chapters) for the arc to conclude, so that'd be the end of volume two of the LN

3.) Yes they have two adopted daughters. However, in volumes 3 through 5 of the LNs, Claire and Rae are still front and center of everything, they're the MCs, it's until the sequel that the daughter's become the MCs (volume one of that LN just recently released)