Learn English – the meaning of ‘is’ in this sentence

The usage of "for one thing" is correct. The meaning of the sentence is the same, regardless of whether "for one thing" is present or not. "For one thing" is intended to imply that there is more than one reason available for liking the tea, whether or not any other reason is supplied in a later clause or sentence. It is acceptable and even common for no other reason to be given in these cases, if it isn't germane to the author's intent to do so. If an additional reason is supplied, it would usually be prefaced by something like "For another thing..." Additional reasons beyond these would probably not be prefaced by "thing", as this would get perhaps a little repetitious.

The passage does not clearly indicate whether it is the lack of spices or their presence that causes her to like Assam tea more than that at home. The sentence, as written, suggests that it is the lack of spices that is more to her taste. Remove the "no" in "no spices" and it would be the opposite.

The de Havilland Moth was a line of airplanes manufactured in the 1920's and 1930's.

Since they are talking about airplanes, and the book was published in 1932, that may be the answer.

According to Wikipedia:

Every light aircraft flying in the UK was commonly referred to as a 'Moth', regardless if it was de Havilland-built or not.