Some questions to improve my battlefield performance

[Korlon]

1) I keep hearing units perform better with guardmode on, is this with all units or just some types, eg hoplite-type spearmen? And when do I turn guardmode off, only when the enemy's line is broken or in some other cases as well?

You would turn guard mode on if you're defending, generally. Turn it off if you want your unit to fight better. Units in guard mode stay in formation, so most of the time the whole front line isn't attacking if you're not defending. Plus, units in guard mode that aren't moving, or if they're phalangitai, lose stamina much slower.

2)Most assault infantry have lower attack values than spear/pikemen, yet their descriptions say they should defeat them. Is this because of higher lethality or some trait they are given? (just to know if it's safe to frontally attack spearmen and possible some pikemen if needed)

Assault infantry generally have swords, which actually means lower lethality, I think. Check the EDU for that sort of information. In this latest version of EB, I believe spearmen have the light_spear trait, which gives them a 4 point penalty to defense against other infantry (and an 8 point increase against cavalry).

3)In which kind of situation do I turn off a phalanx to fight with their swords, and more generally when do I let spearmen fight with swords?

Generally, you would try never to let your phalanxes fight with swords.

4)Does using alt-attack after a Cavalry charge and thus the cavalry using their secondary weapon, make them perform better?

Depends on the unit it's attacking and the weapons the horsemen in question have. You can use alt click to order the charge as well. The horsemen automatically change to their secondaries after the successful charge. Anyways, if the opposing unit has heavy armor, you may be better using a weapon that's armor piercing. Generally, heavy cavalry from the east have an AP lance and a secondary sword, so the lance would be better.

I would think the lance or spear would be better off against cavalry as well due to the bonuses it gives against them. And, of course, use the secondary against light armored troops. Generally, the lance has some waiting time between blows, so it'll kill lightly armored troops slower.

5) other game mechanics i can use but are not widely known (atleast to noobs)

Can't really think of any right now...

PS: whoever created the recruitment viewer, wouldn't it be possible in future version to implement a feature allowing you to , after selecting a faction, to right-click on a province and you'd get to see a list with what units your faction could recruit there and what level/type of MIC would be needed. ( because now you just see how many units become available)

It is possible. Click on the province in question. Click on view in the upper left. Click on province details. Click on detailed info for information on MIC and such.