[Halancar]

Well, I played Marveni a little in the past, so I'll try to answer :

1) Marveni units are well-rounded, and slightly above average. Unfortunately, they are slightly above average in every stat (hp, strength, attack, defense), which makes them expensive (in gold) and also mean they do not have any super unit around which to build a strategy. So you need to plan for a high turnover of troops, and you need a good cash flow. And they are so well rounded that you cannot really build a specific strategy around them.
I would recommend looking for boosting spells (Legion of Steel, to start with) while working on battlefield strategy (a strong center of noble warriors, javeliners around them, and eponi knights on the flanks on attack rearmost).
Magic research : I would go with alteration (boosting spells) and construction (for middle and late game summoning spells). Of course, another strategy would be to go for thaum 1 (communion) then evocation, and start blasting away.

2) That's a hard question. I would recommend starting with research for a few turns (say, 10 or 12) to get the first useful spells. By that time you should have recruited enough mages to keep on the research while your pretender can go site-searching. But it is a hard call to make

3) Build a paper tiger army. You need to get and stay on top of the army size graph, even if it means parking 500 marverni slingers in your capital. As long as you stay there, the AI will not gang on you. Then you can build a real army and pick them off one by one.
Upkeep-free units are particulary valuables for your paper army. Summon animals (Conjuration 2) would fit a druidic theme perfectly. But if you are rich enough, any unit will do.

4) I would say because dead units do not get tired and recover from afflictions.

5) There is only one way to get a feel for each race's stat, it's to play them. And make sure to watch the battles when facing a race you don't know, click on their troops, take note of the spells used...

6) As has been mentioned, you cannot give one rank in astral to someone who does not start with it.