In the examples, ─ is a stand-in for the word being defined. I think this is done since there can be alternate spellings for the word and with the stand-in they get to avoid picking a specific spelling. I have no idea what the ◇ is. I’ve never seen that before.
As for ヱン, it’s probably just listing the old spelling from before the kana reforms where that kana was eliminated and replaced with エ. You’ll sometimes see a similar thing with verbs where dictionary entries will mention that verb is 五段 and that in classical Japanese it was 四段.
I tried to check the official site of that dictionary, but the paper version does not include that symbol. Maybe it’s specific to the electronic version? Anyway, from context, it just means like a way to mark the beginning of a clarification.