If we would use 2 Business Units, would we still have the main Account with access to all lists from Business Unit level and all subscribers with a possibility to mail them all with a specific sender profile?
Not sure I get the full scope of your question, but overall - yes. All subscribers added in the Child Business Unit will also be added to the All Subscriber List in the Parent Business Unit - and thus, if you send an email to the all subscriber list - yes, you would have access.
If your data model is based on data extensions, then this wouldn't be possible from the Parent, given that the parent doesn't have access to data extensions stored in the child business unit.
However, if you use data extensions, you can populate a "total audience" data extension in the child business unit, by selecting data from both the child and the parent bu (i recommend 2 queries), first one selecting data from child, second selecting from the parent by adding ent. in front of the data extension name.
All depends on what you are trying to achieve.