If I understand the question - Also why does radiant superheat decrease with an increase in load? The answer is cooling occurs and heat transfer is lowered from the flue gas side or fireside to the steam side.

If more excess air is used it will cool the furnace and flue gas and stack temps. Superheater temps may also be reduced because of total furnace heat reduction has occured. Pressure should not change unless load changes dramatically. Part 2 of your question, Radiant superheat temps will be reduced as load increases because the flow velocity of the steam has increased and is traveling faster thru the superheater, and cant pick up any more heat. Also other superheater convective will increase in temps with increase in loads and the inlet temps to the radiants may be increased. with these increased inlet temps the temperture differential may be lower.

With fuel rate held constant and air flow increased the losses to stack will increase due to increased flow.
This means less steam will be generated so pressure will fall.
The increased flow of combustion gas with a reduction in steam flow means convective performance will improve so steam temperature will rise.
The air heater will see increased colling from the air but also increased heating from the fuel gas flow so will not change much in temperature

Superheat temperature in radiant tubes will fall because the radiant input is basically unchanged with load so the temperature is high with low steam flow and low with high steam flow.
Designers try to balance the ratio of radiant surfaces to convective surfaces so the temperature stays about the same with load.
ie radiant loses as convective increases.