I feel a contradiction in the rules. On one hand it says: "Bei Nahverkehrszügen können Sie einen früheren oder späteren Zug nehmen". But it also says: "Sie können Ihre Fahrt zwischendurch nicht unterbrechen."
It seems like here it's a matter of interpretation, in you case it's not really a "unterbrechen", but just a stopover. And with ticket like the one from the previous post OeBB basically confirms possibility of such stopover because otherwise it wouldn't be possible to make that 2 hour stopover in Launsdorf (why it doesn't give more time it's already a different question) at all, system wouldn't allow it.
I'm pretty sure if I ask ÖBB they'll play by the rules and say it's not possible to break the trip. I'm wondering how relaxed are the Austrian conductors regarding this issue.
It's difficult to account for human factor, but simple search shows that we aren't the only ones who asked similar question (e.g. drehscheibe-online.de/foren/read.php?106,7229450 where person asked about zugbindung and received an answer that it's only for RJ)
But I'm still wondering if it'll be accepted to take earlier trains and spend a bit more time at the castle.
Well, just found an elegant way to trick OeBB ticket shop and get you more time in Launsdorf. When inserting journey details choose connection time of at least 20 min., this way you can basically move your departure time from San Candido to earlier train while still counting on RJ №630 (as definite zugbindung). It should look like this -->
This way you will ensure that on the ticket it will be written that departure time is 7:15, then in Lienz you "miraculously" would be able to catch earlier train to Spittal (not 9:24, but 8:24), no way OeBB conductor would mind this. Same action for train change in Spittal. Overall this trick will ensure your arrival to Launsdorf at 11:32, while from there you will continue as in the travel plan, meaning at 15:33
p.s. in case 7:15 departure is too early for you, then you can always play the same trick with later departure time (meaning depart 9:15 and then count on RJ №632, while still having 4 hours for Launsdorf)