Both translations: They were so lost in their thoughts that they weren’t even aware of it. and They were so much absorbed in their thoughts that they didn’t know what they were up to. are very correct.
No, we can not put a comma
,before or after tant.
No, as the phrase is, we can not add que to tant here. The original phrase can be reworded as follows:
Ils ne s’en apercevaient pas tellement ils étaient absorbés dans leurs
The implicit que you are talking about can be used under the condition you change the position of MC and SC as follows (
MC + tant ...que + SC):
Ils étaient tant absorbés dans leurs pensées qu‘ils ne s’en
Ils étaient tellement absorbés dans leurs pensées qu‘ils ne
s’en apercevaient pas.
Additional information: tant is called adverbe corrélatif and it expresses the intensity (as in your example) or quantity (in some cases)