I found in CSS610-8G-2S+IN switch specification that it has 2 DC inputs (DC jack, PoE-IN). Is it stay online if I connect DC jack to one electric line and PoE injector to another one and one of electic line will be disconnected? Or switch will be rebooted in this case?

Yes, they do provide redundancy. Just keep in mind that it will always use the highest voltage source. E.g. it'll keep consuming 48V PoE even with DC present if it's just 24V. This might not be desirable in all situations, like with limited PoE source budget.

Additionally, since it specifies Passive PoE i'd probably use simple injector or forced output. Otherwise it might detect no-load condition and disable PoE if your e.g. power your 610 with higher voltage DC, like 57V.