Benadryl does crap IMO. I've tried it at high doses, and nothing. Ketotifen is much more effective.
ECA and Clen will work on the same basic principles, so it would not be a break for your beta receptors. Drop the ECA.
You can keep running T3 while you're off clen, that shouldn't be a problem.
While you're off for 2 weeks, you can also add in something like Alpha-T2 to hit the Alpha-2 receptors to stop fat from being preserved. Just be sure to dose it at least 30 minutes before meals, insulin will kill the effects.
When you're done with the clen/T3, be sure to do some thyroid PCT with something like 7-keto DHEA and kelp (iodine). Your thyroid will recover, but during the down time you'll be tired and metabolism will be down. You want that to recover ASAP.