You shouldn't change your outer ring only to a 53t as it is unlikely that your front derailleur will cope with the larger gap between the rings. Most front double derailleurs will only accept a 16t difference, going from a 50t to a 53t will push the derailleur to 19t difference.

A and B rings can be mixed although it is better to keep them together as both A's or both B's, this increases shifting performance.

You could fit a single ring to the cranks and run a XT cassette at the rear if you wish. You don't have to use two rings on the cranks.

Unfortunately we currently do not stock an inner chainring in the shimano 7800 model.

Shimano designate two different chainring types to be used as sets for optimal shifting (though in practice you can still mix and match and get excellent shifting);

Chainrings denoted as type A are for use with a 42 tooth small ring, and the matching 53T A chainring.
Chainrings B are for use with a 39 tooth small ring, and either a B type chainring 52T or B type 53T size.

your chainset has a 130pcd chainring size so you can select any chainring tooth size that also has a 130pcd specification. If you wanted easier gears you would perhaps need to consider switching to a compact chainset (50/34 tooth).

These would be ok for use with a 7900 chainset.