This is a really good point! In most narrative media, you wouldn't get a T-Rex transformation reveal twice. Most authors would just do it once, for the impact shot, and then move on to something else. But I have absolutely been in a D&D fight where the best strategy was to try repeating That Thing We Did Two Turns Ago That Almost Worked. Fun to see it in comic form!
It's also still a good storytelling moment. The vibe I get from a second transformation (aside from the tactical problems for Calder) is that Calder is losing his ability to control the battlefield. He tried to shut down the T-Rex and it didn't work. That feels very much like when I'm running a boss monster and it tries & fails to stop a PC's spell or effect. It's a real blow to the boss's ego usually, and indicates that they are on the back foot.
Yeah, I agree. The Oracle's vague teasing about "and as for the elf..." WAS cut off by Belkar's attack, which would be a decent way to hide a major spoiler. But given that the Oracle said "yeah I wasn't buying any of those either", he could just as easily have finished that sentence with "Vaarsuvius will die of Elf Old Age 2.5 years earlier than they otherwise would have, given all the stress your shenanigans put them through."