It isn't at all obvious to me why a true "sublease" which creates a legal property right in certain defined real estate in exchange for some amount of rent to the exclusion of occupancy by the primary tenant, would really be necessary in the situation that you describe.
One can grant someone permission to use your space, and even charge them money for doing so, without creating a property interest called a "sublease".
For example, when you go to a movie theater you pay money and are given the right to sit in a seat (in the fancy movie theaters these days, even an assigned seat) for a particular period of time, but you have not subleased that assigned seat from the movie theater. Instead, you have a "license" to use that seat and are either a licensee (or more likely an invitee) of the movie theater.
Assume that the rented space would essentially be a regular home
office, would never involve physical business meetings with more than
a couple of people a day (or even a week/month, e.g., not an
in-and-out situation), and would not be a mail-related business.
If above is totally OK, would the situation be any different if you
were to have an extra 11:00 to 19:00 employee on the premises?
Normally a residential lease will contain provisions stating what uses of the leased property are allowed and what uses are not permitted which would have to be interpreted on a case by case basis. If there is not stated list of permitted uses, generally "any lawful use" of the property given its current zoning classification would be permitted.
Most residential leases and most zoning statutes in places zoned residential would not prohibit the use of a premises as a home office used only by the resident and an occasional visitor (assuming that the business activity was not itself illegal like drug dealing, money laundering, loan sharking, prostitution or pimping, etc.). But, it wouldn't be unheard of to see a residential lease or a zoning ordinance that did prohibit home office uses.
An eight hour of day employee on the premises starts to look like a commercial office use, which many residential leases and many zoning statutes in places zoned residential (that would apply in the absence of a more expressly stated lease prohibition) would prohibit. But, again, only the particular lease in question and the particular zoning ordinance applicable to that particular location would matter. If the lease said, "any lawful use" was permitted and the property was zoned for either commercial or residential use, hiring an employee to be on the premises wouldn't violate either, even though a true "sublease" might still be prohibited.
