I adjusted the meaning a bit to accomodate the newer usages. My dictionary lists nouns without the である. With the adjectives, it doesn't use である.

There does appear to be one or two other definitions for にすぎない which aren't listed here, so those formations might be coming from those?

I'm using the 'A Dictionary of Intermediate Grammar' and the advanced ones - these are pretty much (from everything I've seen) the standards for grammar reference - they have more data can you can possible absorb, so I'm inclined to trust them when confirming data. It's probably that you can put whatever you want after a な adjective - whether is's な, だった, or である.

I use the second one (although I left it in my boxes in Japan, sniff - could only take home so much), it's a good book. It's not my intention to say one is right or isn't right, but I've ultimately got to pick one. I've seen lots of books approach the way of marking usage patterns very differently. I like the aforementioned dictionaries because they tend to give the most basic way of describing a usage, and often (but not always) leave it up to the reader to realize that the forms can often be mutated to handle tense and/or politeness.