Q-1) Should we do seva kalam pasuram chanting on varshabdhikam day or next day abdhika thadhi aradhanam day?
Sevakalam is a part of Thiruvaradhanam which (Thiruvaaraadhanam) is a part of Shradham and which has to be done every day or at least on all important days like festivals, vrathams, shradham etc.
And actually it is not coming under shastras but in Sampradhayam because it differs among brahmin community.

It can be done on the Dhadhiyaaradhanam day too as Thiruvaradham is a part of every day which is not in old custom but people are slowly shifting this to Dhadhiyaaradhanam day to hassle free on shradham day
and to entertain the participant of Dhadhiyaaradhanam.
I could not totally disagree with this but worrying about the breaking of custom where chance of many other unknown benefits may also be there.
I recommend an Upanyasam can be kept on Dhathiyaaradhanam day which will be better to entertain and understand many things about the rituals.

Q-2) What pasurams are usually recited?
Vk = Vada Kalai; TK = Then Kalai
VK Madam: Angan, Kaasai, Vanilaa
VK Muni and TK : Amalanaadhipiraan

Kanninun Siruththaambu
Koil Thiruvaimozhi (Thiruvaimozhi ful can be recited if started in the previous day)
Ramanuja Nootrandhaadhi
VK: Desika Prapandam
TK: Thiruvaimozhi Nootrandhadhi (optional)
TK: Upadesarathnamaalai

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Q-3) Can the next thadhi aradhanam day be moved to next weekend for convenience of guests or does it have to be done the day following varshabdhikam
Can be moved to any nearest day according to convenience.