Making rules text both clear and concise is a difficult and underappreciated skill, but it's very valuable, especially when you're trying to fit the rules in a small box on a template, such as for cards or hero abilities. I changed or dropped several ability ideas for my hero editor because they didn't fit in the box.
One important technique is to re-use words and phrases that already have an established meaning; for example, saying "when you place a Rest order" or "when you recover your fatigue by Resting" instead of "when you perfrom a Rest order." The word "place" is already used consistently throughout the rules and cards to refer to a hero gaining an order token, and the phrase "recover your fatigue by Resting" appears (verbatim) on an official card (Red Plate of Kelos). By usine one of those phrases, you signal to the player that this ability works like something that they already understand (and which thing), rather than in some weird new way they have to figure out.
Similarly, you probably want to say "figure" instead of "player [hero] or monster", "move" instead of "hook", and "(ignoring armor)" instead of "directly" (when talking about wounds). Because those terms are used by other rules and abilities, so players (or at least, experienced players) already know what they mean.
Another technique is to rely on the game's established conventions to convey rules implicitly, instead of spelling them out. For example, abilities in Descent can be used an unlimited number of times by default; you need to specifically state something like "once per turn" or "exhaust" to limit them. Therefore the entire sentence in Shendelzare's ability stating "this can be done multiple times, paying its cost" is unnecessary. It's sometimes nice to spell it out, if you have the space, but if you can't fit in the actual important rules, that's something you can cut to save space.
Sometimes you just have to simplify how an ability works in order to fit it in the allowed space. For example, you might limit Shendelzare's swap so that it only works on 1x1 figures, not for any game balance reason, but just so that you don't need to explain how it works for larger figures. You might eliminate some of the numerous restrictions on Chen's ability for similar reasons.
But other times you can keep a more general or more complicated version by finding just the right subtle turn-of-phrase. There's a skill in Enduring Evil called Necrotic Traversal that teleports you to the location of a figure that just died; I was able to clarify large figures by adding just a couple of words to the text, saying "move to any space occupied by that figure" instead of something like "move to that figure's space." But that only covers the case of a small figure moving into a larger figure's footprint, not vice versa.
I'd also like to comment that I think it's somewhat unreasonable to expect custom heroes to be balanced both for vanilla and the extended campaign, especially coming from someone who's only played vanilla; lots of stuff has to be changed or removed in the extended campaign. Pointing out which stuff would be problematic in RtL is certainly useful, but I'd recommend focusing on vanilla, at least to begin with.
Darkspell's suggestion to heal when a glyph is activated resembles an ability from the build-your-own-hero rules: "Each time a glyph is activated, you may restore up to (3-5, depending on ability level) wounds, split however you like, between yourself and any other heroes within 3 spaces of you."