Tact and Diplomacy is almost as broken, but recall it only works when fighting at cities, and not at home realms. No, it's major benefit is the ability to snipe high cost cities from the central play area for low strength, and then keep them without much trouble.
You are right about the rest, but I am not certain about the Tact and Diplomacy Rune. The way it is written, alongside with the text of other cards, I assume that "During a battle or siege against a city" means that it can be triggered only when you siege the city, and not when you defend it. Otherwise they write it like this: "When you participate in a battle or siege of a city...".
As for the Death and Decay - I think it is alright, you can start from turn 1, if you wish, to attack home realm with a 1-cost unit, pass and deal 1 damage, alongisde destroying cards you do not want. 5 turns - 5 damage against the strongest player, filtering those coins and basically changing your 5 1-gold coins to higher value gold. The downside is that you are basically losing 2 cards per turn to run this and your oponent gets to cycle his cards... It sure has it's uses, especially for races with more card fetch.
My concern is Wrath and Vengeance - defending the Home Realm is a primary task, and to utilize this card you have to spare a card in your hand, taking the 33% chance that your oponent will roll no skulls. Seems too random to me. An idea you can try - modify the ability like this: "...and destroy one card from your hand, if any, to...". This way you can dump your units to the battlefield to minimize the incoming damage if your luck is bad while still destroying any obsolete gold/ tactics that slow your deck down. Use this carefully though, as in addition to minimizing the early aggression, now it will be awesome in the endgame as well.
Finally, Speed and Stealth. I have no idea what to do with that. The thing that troubles me is that 4 of the 6 cards are features you can choose to use. However, Wrath and Vengeance requires someone else to do something so you can use it, and Speed and Stealth gives you the bonus AFTER you fulfill a condition - so basically it is not a rune, but a quest. Therefore it is good to think of house rules that makes them activate by the players choice to increase their potency. My idea for Speed and Stealth is this: "Whenever you defend successfully in a siege, deal 1 damage to the attacking player's Home Realm. Do not exhaust this card when you use it." Thus we keep the spirit of the card, being a counterattack, and now it is actually good - makes the opponents think twice whether they should attack you.
Here is the place to admit that I still haven't played Runewars scenario, besides testing it alone. However, some of the effects on the runes are quite obviously weak as you too have observed. When I manage to gather my gaming group and play several times I will share my observations.