How are the seven full mouth letters to be pronounced when a sukoon appears on top of them? For example, how should one distinguish between "Us" (where the "s" is saad) and "Us" (where the "s" is seen)? Is the "U" supposed to be pronounced full mouth?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The way to pronounce the letters of Isti‘laa’ (emphatic letters) is by learning them from a reciter who masters the rules of Quran recitation.

As regards knowing how to pronounce these letters in theoretical terms, it becomes clear by knowing the definition of Isti‘laa’. The author of Fat-h Rabb Al-Bariyyah, a Commentary on Ibn Al-Jazari's Muqaddimah explained it by saying:

“Literally, Isti‘laa’ means raising; conventionally, it means raising the tongue to the upper palate when pronouncing the letters of Isti‘laa’, which are: خ (Khaa’), ص (Saad), ض (Dhaad), غ (Ghayn), ط (Taa’), ق (Qaaf) and ظ (Thaa’). One can memorize these 7 letters in the phrase: خُصَّ ضَغْطٍ قِظْ (khussa dhaghtin qith).” [End of quote] 

As regards differentiating between ص (Saad) and س (Seen) when pronouncing them, with Sukoon and in all other situations, it is by knowing that ص is characterized by both Itbaaq and Isti‘laa’. Itbaaq means to "cling together"; and here it means to cling the tongue together with the upper palate when pronouncing the phoneme (letter) in such a way that the voice becomes trapped between them, which is why this feature is inseparable from the letters that have it. The Itbaaq letters all possess the Isti‘laa’ feature, but the feature of Itbaaq cannot exist without the feature of Isti‘laa’. This is why the scholars of Tajweed mentioned that the Itbaaq letters are distinguished from all the other Isti‘laa’ letters with extra tafkheem (velarization).

The Saad is different from Seen, as the latter is characterized by both Istifaal (lit. 'lowness') and Infitaah (lit. 'openness'). Istifaal means that the tongue will be in a low position. You will have to move the tongue but not all the way up to the palate of the mouth. Infitaah means to keep the tongue separated from the roof of the mouth while pronouncing a letter.

As regards the ء (Hamzah, to which you referred as the "U"), it is not pronounced with Tafkheem (velarization) in the above two cases (i.e. Itbaaq and Isti‘laa’) nor in any other case, because it is among the letters of Istifaal. The scholars mentioned that the letters of Istifaal are all low letters and it is not right to pronounce any of them with Tafkheem with the exception of ل (Laam) and ر (Raa) in some cases, and the ا (non-voweled Alif) if it is comes after a letter that is pronounced with Tafkheem.

Ibn Barri said: “The letters of Istifaal are all low letters and it is not permissible to pronounce any of them with Tafkheem with the exception of ر (Raa) and ل (Laam) as there is elaboration regarding these two letters.”

Allaah Knows best.