You've spelled out basically all the information I was able to find on the matter, and the rules seem to work exactly as written. The only source I found with more information was Arcana Exxet, which is more detailed on how Sacrifice works:
There are spells and mystical abilities that allow the caster to voluntarily
sacrifice points of life to gain certain additional effects or otherwise
enhance their power. These cases, unless specified otherwise, are
subject to the rule of Sacrifice. This means that Life Points lost in this
way are not regained in the conventional manner; they ignore natural
or mystical regeneration and the effects of spells, powers, and healing
techniques. Sacrificed Life Points recover at a rate of 10 per day (in the
case of creatures with Damage Resistance, this amount is adjusted by the
Multiple of Damage Resistance) regardless of other special conditions.
Arcana Exxet Chapter 10 "Option Rules" pg. 118
Spiritual Damage is handled just like normal damage: That is to say that you handle attack, block/dodge, armor, damage barriers, and damage reduction all normally. Then, you would write the resulting damage on your sheet with some sort of annotation to make sure you remember that it has special healing rules. (I would be tempted to use the * symbol next to it, from my days of WoD)
So when it comes to negating the spiritual damage, you have all of the normal options available to PCs: Intangibility and Damage Reduction from the Nemesis Abilities, Damage Barrier from the Ki Abilities, any number of Shields from Magic and Psychics, and normal everyday Armor are all great choices.
Once you have the damage on you, however, it's much harder to remove than normal damage. No healing effect I've seen overcomes the Sacrifice limitation (and it would have to be very explicit for me to allow that), and Sacrifice is very clear that Sacrificed health heals
independently of the character’s natural Regeneration or any healing spells he might employ.
One nice thing about Sacrificed HP (and also Spiritual Damage) is that because it's completely independent of other healing, a character who has taken mixed damage (both "normal" damage and Spiritual/Sacrificed damage) actually heals faster than a character who has only taken one or the other!
It turns out that there is at least one way to (kind of) heal Sacrificed health: the Judgment Arcana in the Invocation Summoner tree. Judgement says
When Invoked, Judgement will undo any event that has taken place
up to a maximum distance of one mile from the invoker. No time change is
Involved, rather reality itself is transformed. An injured party's wounds could
be healedn for example, and even the dead can be brought back to life if they
were killed in the time period transformed. It is not possible select among
events in the area and time period - everything that happened is undone
Anima: Beyond Fantasy pg. 186
Technically this doesn't heal you of the damage, it prevents the damage from ever being dealt to you. Functionally this acts like healing to the character, but in general it can be dangerous to alter reality in the Anima universe. Because of the nature of Anima's reality, there is likely either someone (or something) of great power with a vested interest in reality staying the way it is, or someone much worse with a vested interest in changing reality a lot. What this means in practice is that the GM is able to make any sort of fallout he would like come from Judgement-ing away a wound. Maybe it didn't happen to you because it happened to an ally instead, or maybe the bad-thing ran away instead of getting slain. Or maybe you didn't sacrifice those LP, and now you lost that fight.
Places I never got a chance to look at, but have doubts they'll help: