not being negative nelly, just sharing some experience as ive been involved in wowies for many years...
smaller male does not nessicarily mean everything will be ok...
my families chihuahua, 5lb female with a then 2 1/2lb male (now 3lbs since neutering 2 yrs ago)
followed her lineage back over 8 gens, free whelping lines, small pups ect...
followed his lineage back 10 gens, small pups and free whelping lines...
no c-sections in either side...
vet check, and xray prior to breeding gave good pelvic width
xray and ultrasounds during pregnancy to make sure the pups were not getting overly large and keep track of how many to expect...
1 breech puppy that got stuck and a c-section/spay later and thankfully all 4 pus were delivered safely...
you said you have "plan b" make sure that plan be is aboslutly secure and have a lan c just in case (remember normal vets arnt open at 4am so make sure to add an extra $1000 or so to the emergency fund in case you need to go to an emergency clinic, costs are always higher at e-clinics.
( had her spayed during c-section because 1 c-section in wowies typically means any future litters will need assistance)
as a side note, the largest of the pups, the one that got stuck, is only 4lbs full grown. shes still smaller than her mother and yet caused such a big issue.
you are right, whats done is done.
hopefully 1 litter is all you need and youll get her spayed and i realy do hope everything goes smoothly. little dogs are incredibly tought to breed.