Sollir: Might be a good idea, even though it still leaves it kind of "open" for Liches to gain large numbers of HP. Liches, as most spellcasters, are fairly powerful compared to HD. It could even be their HD with no modifier that could be the cap, still would make them able of finding "occasional" targets.
I even considered, taking into consideration the power the Lich already has, that the they should be temporary HP instead that vanish at a rate of 1 pr. day or something. Would make a good adventure opportunity. A Lich that goes around slaying whole populations in small villages, in preparation for a showdown with an ancient enemy of his.
Marauder: Well, It is a undead being, evil and often somewhat insane. If a church should have any one creature they would like to perish, it would most probably be an ancient Lich. Yet, of all undead creatures, the Lich is the one that had the most control over its transition to undeath. Therefore, it would be logical that those that created the ancient ritual of Lichdom made it so that the Lich would have some protection against that which haunts most undeads, this being good clerics & paladins. Especially the Aura of Divine Hatred affects those two.
And yes, a Category 7 Lich would perhaps be able of taking out a Tarrasque, if he wanted to that is. Yet, a Category 7 Lich would have to be an Archmage with at least 5000 years behind him, which should allow him some power, even if he wasn't a Lich. The last couple of categories are intended for Epic play...