Does Svellsongur copy text that the unit has gained from other sources, such as the Assault granted by Cleave? What about text it’s gained from other Equipment?

No, Svellsongur only copies the text that’s actually printed on the unit. This is an ambiguity in the current rules, which refer to “appending” and “removing” rules text without reference to whether that text is original to the unit or not. The key word here is actually copy, a system that will be fleshed out in a future rules update. For now, we’ll just tell you that copies of objects or text take on their original traits, without regard to any subsequent alterations. Svellsongur doesn’t grant the unit additional “copies” of the abilities of other Equipment. So putting 3 Svellsongur on the same unit will give that unit four total instances of its abilities, not eight. (Four should be plenty.)