Is this untenable in an au pair set up to seek evening help if it is over the 45 hrs?
Yes.
or is it feasible to expect more than 45 hrs if i'm willing to pay OT?
No.
our needs are more like 8am-7pm
No.
or more daily
No. No. No.
or will i be shut down immediately by the agency if i suggest i would like more than 45 hrs?
Hopefully.
What if i did some part time daycare for the infant, in order to free up some of the AP hours. Then we could use her more in the evenings or mornings?
That might work. If you were going that route I would suggest putting the infant in daycare in the mornings. A parent drops them off at daycare on their way to work, AP picks them up later in the day (say noon-ish).
AP could then work 12 - 8/9 and have mornings off.
(It won't be easy to find a candidate that is happy working until late at night, that's usually when they want to meet their friends, go to the movies, go out for dinner, most of their friends even on a split schedule will work a few hours in the morning and then be off in the afternoon or early evening, your AP's social life will suffer and with that you are in for lots of resentment)
Or am i still overestimating the AP program?
Yes. Even with adding daycare.
I think the evening help would be more flexible and informal, the AP would be invited to have dinner with us anyway, etc.
You can "invite" your AP to have dinner with you as much as you want... remember that invitations can be declined. With an infant and a PK3 child your AP is much more likely to be out the door the minute she is off.
If you expect her to have dinner with you to help with the kids (or to watch the kids while you fix dinner) she is working, not "invited to have dinner." As soon as you start with helping with bath time or putting the kids to bed, you are "inviting" her to work off the clock. As soon as you expect her to be around and help, no matter how informal, she is working. An AP would have every right to be resentful and ask for rematch because their HF is breaking the most basic of the program rules in that case.
As everybody else has said - move on.
I am sure you could find ways to make it work but the way it is an AP couldn't even cover your base hours if you anticipate "needing care for the infant from 8am to 6pm" on a Mon - Fri schedule (50 hrs/week) and as you'd already be at the daily limit (10 hrs/day) with your regular schedule, you couldn't ask for even an extra minute on a regular day. The program simply doesn't sound like a good fit for your needs. Though I have to admit that I can't think of anything that would. Even a regular nanny doesn't sound feasible if you need "8am-7pm or more daily" (and I dearly hope that "daily" means Monday to Friday and not 'every day'), a nanny appreciates a schedule that is not "7 pm or more" and you won't find too many that will work 8 am - 8 pm every day. You can't expect one person to work 60 hour weeks.
To make the program work for you, you'd have to make changes to your lives that might be too complicated to make sense (stagger working hours, work less, work from home while watching the baby etc.). The program is much easier if it already suits your requirements or if your usual routine only needs minor tweaks, not if you need to make major changes to your daily schedule to make it work somehow. With your needs you are running the risk to burn out even a rock star AP and neither your family nor your AP will be happy in the end.