If I restore my iPhone 3GS to original firmware via iTunes, will it remove the iPad baseband I have on it, and became a default iPhone baseband?
I just want to get my iPhone all clean, without hacks.
Yes, restoring the iPhone to factory settings will make it so that the phone is as good as new, which includes restoring the iPhone baseband from the iPad baseband.
yes if you restore and update your iphone.
No it absolutely will not. You will get "erro 3194" because Apple will not recognize your SHSH blobs because your phone is using iPad baseband. You need to downgrade your baseband back to normal using Redsnow.
yes it will
It will do a complete factory restore, which reverts to the phone baseband.
Actually, if you have jailbricked you iPhone, it won't let you restre it through the normal process. You will have to download the ios version you want and manually install it like you did the jailbreak version
It will be restored to a phone baseband. then you will have to jailbreak it again.
Restoring your iphone will update the phone to the latest software in the process it will update your baseband and firmware if there are any changes done to them. If you have the latest software then it wont change it.
That is correct - if you restore your phone within iTunes, it will become a standard iPhone without any modifications to the baseband or software. You will lose your jailbreak, any Cydia apps and of course any baseband you are trying to preserve. If you have unlocked your phone by using the iPad baseband then you will lose your unlock.
Remember that Apple do unlock phones, which is also done within iTunes. You must make a request to your carrier, and in many cases they will try and delay or put you off doing this (speaking from experience with Orange UK) but perseverance is key. If you live in the EU they have to do it by law, which is great though in the US, Australia etc... I'm not too sure about how this works.
Regardless, it can be done and you should definitely look into it if you need to unlock your phone. I do know that certain carriers will take an exception if you are not the registered owner, though this might not be a problem for you.
Don't forget restoring in iTunes will also update your phone to the latest version of iOS, which is iOS 6. You'll get Apple Maps, Passbook and so on though you might take a slight hit in terms of performance. Here's an ArsTechnica article all about iOS 6 on the iPhone 3GS: http://arstechnica.com/apple/2012/09/tempting-fate-installing-ios-6-on-the-iphone-3gs/
Good luck, and let us know how it goes.