What does Maugham mean by "his spaghetti were"?

I'm pretty sure it's number two. After all, Spaghetti is an italian plural, no matter what us italian-cuisine-importing countries make of it :)

A similar case is Zucchini. It is the plural form of La Zucchina.

I find it difficult to extract a precise meaning from the sentence, assuming that was is written is written correctly.

his spaghetti were (plural)

brought in a huge dish of it (singular)

we ate it (singular)

To me the sentence's meaning only works as

I confess that his spaghetti was very much better than his pictures.

I think it is highly unlikely that either 1 or 2 is correct. I think Maugham has made an uncommon[1] choice to make spaghetti plural. Or if you think the speaker is expressing an opinion, he could be using the subjunctive.

[1] COCA & BNC searches yield no such usage.

I don't think this is especially complicated. The phrase "his spaghetti were..." contains an ellipsis, or perhaps synecdoche. More fully he means "his spaghetti meals were..." With such a ellipsis you would normally use a plural word, which would arguably be: "his spaghettis were..." but he chose to simply use the Italian plural instead.

Plurals moving from one language to another are often a subtle and arguable thing, and sometimes I think some skilled users of English are rather to pedantic about it. For example, it is not uncommon to hear phrases like "the data are", rather than "the data is". In some translations of the Bible you see the plural of Cherub and Seraph given as "cherubims and seraphims", which use both an Hebrew and English plural. And it parallels the old argument over whether a single cube is a dice or a die.

My opinion is that both are right, though I think that spaghetti is now sufficiently incorporated into the language that spaghettis would be more appropriate. It is worth pointing out that the passage was written nearly 100 years ago, and perhaps spaghetti was much less common a word in English at that time, and the Italian form might have been more appropriate at that time.

I'd say spaghetti is commonly used as a mass noun, but it is hardly unprecedented for a mass noun to have a plural too. Fish and fruit are both mass nouns, but fishes and fruits are both perfectly acceptable, if subtly different in meaning.