First of all, about 割の合わない - 割に合う is a set expression meaning "to be worth it". Literally I think it's derived from the use of 割 to refer to "profits", so it essentially means "to match up to the reward", but it's easiest to just think of it as a set phrase since it comes up far more often than this use of 割 in any other context.
As such, something that doesn't 割に合う, ie. a 割に合わないもの or 割の合わないもの, is something that "isn't worth it" - the reward isn't sufficient to be worth the effort expended.
Now for the ほど, this should be easier to understand if we remove all the fluff in the middle of the sentence. The basic sentence we're looking at here is 信用ほど割の合わない投資はない。 This is a case of the common usage of ほど which takes the basic form XほどYなものはない, meaning "there's nothing as Y as X" (eg. 自然ほど美しいものはない, "there's nothing as beautiful as nature").
In this case もの is replaced by the more specific 投資 meaning "investment", so instead of "there's nothing as Y as X" it's more specifically "there's no investment as Y as X". The X is straightforwardly 信用 "trust", and the Y is 割の合わない, making the whole thing mean literally "there's no investment as not-worth-it as trust". But that's not very natural English, so let's flip it around a bit and say "Trust is the most unwise investment of all."
As for the bits inserted in the middle of this sentence (積み上げるのに苦労して、失う時が一瞬なんて), they're just elaborating on why the author thinks trust is such an unwise investment - because "it takes hard effort to build up, but you can lose it in an instant". You can think of this whole phrase as loosely modifying 割の合わない投資. It's the same usage of なんて as in something like あんないい仕事を辞めるなんて、馬鹿なやつだ。 ("What an idiot, quitting such a good job.") The whole phrase preceding なんて is commenting on the negative context that justifies the following statement.
Honestly, I don't think this is a very grammatically satisfying sentence, because the なんて clause is completely disconnected grammatically from the ほど～ない expression that it's embedded in. It would look a lot neater to me if the なんて clause was moved to the beginning of the sentence to avoid breaking up the rest (ie. 積み上げるのに苦労して、失う時が一瞬なんて、信用ほど割の合わない投資はない。) But at any rate, that's how the sentence fits together.