@1. NO this is PolRegio=local train (IF it still is-if it is now Malopolskie then NO pass valid). ANyway-I assume you do a daytrip and the local fare is much, much lower as the cost of 1 passday. Most people simply book a minibus in that hoStel in Krkw-ad that is likely indeed the more easy way.
@2. depends on what you want -seat or sleeper. Again I see you stay a few days in Krkw (which is indeed a beautiful city with lots to offer the toruist) so just do it right after arrival there-go to a window/counter! Anyway, I am not sure if can do online, perhaps via cd.cz, and then maybe, perhaps 2 month advance-but shorter is also possible
@3. try now or asap via nightjet.com. This train as such does not daily overnight.
About any newbee here will also get a welcome from anna b fra Sverige or others with a link to a long list-well, you must have seen it-and this has an overview to where best to RES it all. Terms are not that fixed-most railways put on some weeks in 1 go some time-or later when there are works feared that may cause the need for different routing or timing. Do not sweat-this is not RENFE and not super-over-busy routes nor times, you will be OK even if you do it 2 days or even 2 weeks after they open. Y buenes viajes.