Hello Gavin,
Resurrection cheat:
When you've been killed, and your obituary has been displayed, you'll automatically be forwarded to Main Menu (New Game, Load Game etc).
Once you're in this Main Menu screen, keep the "W" key pressed, and at the same time briefly press the "R" key. You should then be returned to the place of your death, with your health fully restored.
Becoming king:
Well, when you've forged the large tribal alliance against Rome, Heremod, who wants to become sole leader of this alliance, will propose to overthrow Ortwin at the first good opportunity during the next couple of months. Since the Cheruscans would then have to elect some fellow tribesman as new king, Heremod offers to give his (armed) support to your candidacy for this post. (He won't make this offer if you've slain his top henchman Wigbald.) Heremod's support would result in your becoming King of the Cheruscans.
However, all of this action would necessarily be stretched out over many months, possibly one or two years - so you don't get to actually play it out, you are just given the perspective and (in your obituary) told the outcome.
From the author's point of view, part of the problem lies with the "historical, realistic" RPG concept: Reaching fuzzy political goals, like removing and finally replacing a political rival, is a long process, and your way to archieve them would have to consist of thousands of tiny moves, a few of them every single day, over a long period of time (witness Gordon Brown's endless quest to replace Tony Blair).
In contrast, if this were a regular Fantasy RPG, I could simply have stated that in order to become King of the Cheruscans, you'd need the Crown of X, and also to fullfill the Quest of Z and so on - and after a couple of weeks and a few spectacular, clearly-defined actions, you would have reached your goal (and in doing so would have had a chance to explore a large additional game world, slay countless monsters, and so on).
Rome's ally campaign too short:
It is. This is mainly because an alliance with Rome brings so many advantages to almost everybody on every side: The Romans will certainly be glad if you join them voluntarily and they won't have to spend all the money and effort necessary to subdue you; so they'll welcome you with open arms. And for your own tribe, such an alliance would bring peace, prosperity and power. The only losers would be some chieftains who'd miss an opportunity to win war booty and glory. So basically, there's not much resistance to be overcome - some of these Germanic warlords need to go, but all other conditions are already in place.
Of course that's not satisfactory from a player's point of view. In retrospect, I do understand that I ought to have added at least several more warlords or would-be chieftains who oppose the alliance and who you'd need to defeat or convince. And I've also shied away from implementing your future life as Rome's ally into actual gameplay (due to the huge amount of research it'd have required to make this more or less historically correct). If I were to start this project all over, I'd do this nonetheless, in order to give this storyline more substance and volume.
Part of the conclusion I've reached is to focus on writing Fantasy RPGs instead of historical or realistic ones in the immediate future: Writing and implementing a historically authentic plot takes at least 20 times the effort and time that writing a Fantasy plot would take, and the same goes for game world creation. Besides, realistic games are far too interdependent: Whatever you do at the Marcomanians' will have to have repercussions at the Cheruscans (that means a great lot of additional dialogue passages to be written); in contrast, in a Fantasy game, the Dwarves of X needn't care at all about what happens at the Elves of Y. So with fantasy, I can very easily offer players a huge game world and an (almost) endless plot, and I can also twist the plot any way I want to, thus offering countless subplots, spectacular events, and real, concrete action and outcomes instead of just fuzzy future perspectives. With a historical game like Teudogar, this is unfortunately far more difficult.