You have found a very interesting problem. Our code book and lesson book do not describe a colon being used in this manner. However, as you discovered, example (9) in §27.f does show this use of a colon. Although the colon is not the topic of 27.f, I would go ahead and follow the spacing used there. That is, when a colon means "such that" do not space it (even though it is spaced in print and even though it is described as a sign of comparison).
Regarding the use of the English-letter indicator within grouping signs, §28.a is clear. When an English letters in regular type is in direct contact with both an opening and closing sign of grouping, do not use an ELI. However when the letter is in direct contact with ONLY ONE sign of grouping (opening or closing) imagine that the sign of grouping is not there and then apply the proper rule from either §26 or §27. §26 explains when the ELI must be used; §27 explains when the ELI is not used. There are many examples in these sections which illustrate these rules.
Thank you for your questions.