"The letter from Farrisia's father contains the following: Expression of great happiness that she's okay, further happiness that she's realized her mistake and is open/eager for reconciliation (there was and is no problem on his end), and yet further happiness that she's actually become the Bearer of Kindness (He has no trouble believing she's worthy of it/acceptable to it, but who would have expected her to _find_ it?). Her mother would be proud. He is, however, worried about her with all the danger she's been getting into and can be expected to, as a Bearer, get into in the future.
Unfortunately, he doesn't know how the rest of the family are doing or where they are; after the incident where she left, he felt he had to tell them about his nature. The resultant debates, arguments, bits of cognitive dissonance, threat assessments, character judgements, etc. combined to give a far better result than they could have, at least: the family left him in Crystal Cliffs and told him not to follow them. He realizes that this likely pains her and that she might blame herself but stresses that a: it could have been far worse, b: as she mentions in the first letter, if it hadn't happened, she probably wouldn't have found Kindness or her current companions on their quest to restore and reunite the Elements and save the world, and c: with just the Element, let alone the world-saving, Farrisia is, even already, in a pretty good position to try and heal the rift. It's unlikely to be the easiest of tasks, but he thinks they can do it eventually.
He also mentions, in the vein of family stuff, that she's actually a pure batpony and that he told the Clan this before they left; he doesn't know if they believed him enough to enter it in already, though.
He's afraid that he doesn't know much about the _Harmony_ beyond its existence; he didn't have much information on that at the time En-Netsusep shattered the Hive, and it was pretty far down the list of knowledge to grab as the connections unravelled. He can't tell her all that much of import about the Catastrophe, either; in addition to the same sort of lack of local knowledge to begin with, he's since discarded a significant fraction of his memories of that time as unpleasant and unlikely to be useful next to what he could replace them with. He confirms, though, that the changelings were definitely not behind it (though he admits that whether they were _responsible_ or not could, at least based on what he remembers, be debated at great length).
He clarifies that he's operating independently, not part of a hive or group, and has been since the Catastrophe. He could probably think of a lot of stories to tell her, but now probably isn't the time. And he does admit that not all of the stories he _could_ tell are very nice; a lone changeling, in over one and a quarter millennia, sometimes has to do unpleasant things to get by, and though he tried not to and certainly never did anything on the scale of sacking a town, he hopes she won't think too much less of him for it.
Regarding why it's taken him so long to reply (he got both her letters), he relates that the situation in Crystal Cliffs has gotten a bit complicated. The meeting she saw him at concerned a leadership dispute among the ponies of the two settlements in which the changelings (and postdragon) had become involved, and, after Farrisia and the Clan departed, the dispute grew rather more heated. It's now been resolved for most of the inhabitants, allowing him access to the postdragon to send this, but it had already escalated far enough that one of the previously even-numbered changelings was killed. A changeling on the same side as the deceased one is still of a mind for revenge. The ponies and other changelings are currently working towards peace (no one else in Crystal Cliffs (who doesn't want all changelings dead, at least) wants more losses among the extremely valuable changelings, and that's before the possibility of violence spilling over into other population segments is factored in), but, as things stand now, if he'd left the settlement to find another dragon, the numerical imbalance might have been enough to convince one or more individuals that swift violence (preemptive strike by the other changeling faction, preemptive betrayal by the other changeling of the same faction in hopes of preventing further violence, preemptive attack on one or more changelings by ponies, etc...) was the answer.
The letter ends with the usual (modified for the somewhat unusual situation) expression of love and wellwishes of familial correspondence."