With purple, I don't generally use my hero's much except for casting spells (for the reasons you mentioned). I notice that your teching Seers a lot (which at a 3/1 for 1 seem really good, until you realise that purple struggles with hand size due to many very good units). I would also delay your Now! techs until after your stronger tech 1's are in/your early tech 2 are in, swapping units out to others on the fly with Temporal Distortion is also stops blue shenanigans (haha, my stinger less hive is now an Immortal!)
Against blue, you really want to go on the offensive. Manufactured truth is great against argo's etc, but remember that his argo's are weaker then yours (no battle suits), that they require a unit already on the field (who will also probs die), and require a hero. Killing their hero/keeping their board clear therefore make it either a) extremely costly in gold + cards (2 cards, 1 + (min 1 for unit to truth) + 2 hero), so even if your hero's are not leveling from them, it should still be a good trade for you.
Against Blue I would also go with slower units. Hive/Void Star/Xenostalker from Future are good, no immediate beneficent they steal/copy it, but with time/now! can be very powerful.