Well when it comes to summoning it's simpler to explain for a cleric. A cleric can not cast spells opposing his alignment, a good cleric can not cast an [EVIL] spell and an evil cleric can not cast a [GOOD] spell.
Summon monster aligns the spell with the monster you summon... summoning a celestial monkey is a [GOOD] aligned spell therefore your chaotic evil cleric may not cast it... his deity just can't grant him the spell. However a fiendish ape is an [EVIL] spell therefore your Cleric can cast that. I believe monsters without the fiendish or celestial descriptor are fair game for anyone. Just as a neutral Cleric could cast both, however casting too many [Good] spells can make a neutral cleric good. Casting too many [Evil] spells can make a neutral cleric evil... neutral is about balance.
It gets a little more confusing for say... an evil sorcerer, an evil sorcerer can summon a celestial creature which is a [Good] act, but then he could use that creature to burn down an orphanage which probably balances out.
That being said... the whole alignment shift is something that takes a long time usually...if your DM realises you're neutral but casting only neutral or good spells...and doing only neutral or good things you might end up shifting up to Neutral Good from True Neutral.
Summary: Evil cleric can only summon neutral or fiendish beasties.Do you have to prepare spells?
Yes. unless you want to fudge it.... but it's more fun with a list of spells, you know what the cleric has as an option available to him. Spells can reflect personality... a torturer cleric of pain will obviously prepare Power Word: Pain and Wrack and Hold person... a Cleric of war will have divine power.
If you need help selecting spells there are a few websites such as
http://d20npcs.wikia.com/wiki/Main_Page - which is an NPC wiki. A quick 1 minute search brings up this cleric:
http://d20npcs.wikia.com/wiki/Cleric_of_Fortune_CR_5As you can see someone else has prepared his spells, which can be helpful, especially for throwaway enemies or checking you've done it right, but with different spells etc.
Now, regarding your story based questions, how much to tell the group etc... They seem very bashy bashy so how much diplomacy do they use? How famous are they?
A party that treats an NPC with a little respect, coaxes out information or has a reputation for getting things done might get the whole story... whereas a party of hooligans might only get what they overhear.
A module should have large portions of text for you to read out to the players. Obviously all modules are different but the last one I ran had large italic sections meaning "read this to players" and then normal sections with info and tables with knowledge and skill checks for more info.
ie.
The adventurers reach the sleeping giant inn, it's getting late yet it is eeriely quiet...like there's no one inside; conversely, the windows flicker with movement.When the players look at the sign they may make a knowledge history check, or similar (lore / bardic knowledge) to discern
DC 15 The sleeping giant inn was the subject of a terrible series of murders
DC 20 The sleeping giant inn is haunted by the victims of the tragic events
DC 25 The sleeping giant inn is haunted by several spectres
DC 30 and the basement is full of ghouls spawned from travellers who sought refuge.
Obviously you only tell your players the italic bits, unless they make the knowledge check or cast a divination spell.
How much would the lady tell them?
Well... she's on friendly terms with one of the adventurers which is good...but it still comes down to 'how do you play her?' a lady might not want to tell the adventurers EVERYTHING due to distrust, paranoia or fear that if they find out how dangerous it is they might not even try.
Conversely she might be exceptionally honest, therefore she tells the adventurers everything she can think of...or everything they ask for...
A lot of DnD is in the hands of the DM, the main thing is being consistant...once you've got a system keep using it. If the players like the style great, if not, adjust.