Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
Unfortunately, I'm almost certain there's no equivalent Dropbox solution for this currently on iOS.
While I cannot give you a technical reason why two-way syncing on iOS is not possible, I can say that at a high level, the iOS platform is more guarded in terms of allowing developers to access certain capabilities of the device.
As you stated in your question, Android devices are capable of two-way synchronization through apps like Dropsync; this is because of the relatively more open-ended nature of the Android platform for both developers and users. To read more on why this is the case, read the 'Android Offers an Open Platform' section of this article.
To confirm, I contacted two customer representatives of two data-sync companies: Loom and SugarSync. I asked both reps the following question:
Is possible for two-way synchronization of files between an iOS device and a computer? For example, if I sync pix from my iPhone to my iMac, and then edit/delete the pix on my computer, will the changes be reflected automatically on my iPhone?
Both times, the answer was:
No, this is not possible due to the closed nature of the iOS platform.
If this capability is very important for your work, I would suggest migrating back to an Android device.
If you want/need to use your iPhone for this, and if you need a platform-independent solution (i.e., you don't want to use iCloud), I'm afraid tedium is in your future. I would suggest that you keep a list of the photos as you edit/delete them on you computer, so that you know which photos have been updated and need to be manually re-download from Dropbox (or manually deleted) on your iPhone when you use it next.
Best Answer
The iPhone takes photos in whatever the orientation of the sensor is then uses EXIF data to instruct proper rotation.
Your friend's e-mail is ignoring (or stripping) the EXIF data. There's no way to fix it on your side.