Should I put her to sleep at 3.5 hr exactly or like 3h 20min and have 10 mins for settling in?
10min is a long time for settling in and TBH, makes it difficult for LO because they can actually miss their window if PD too early. I'd go for PD about 3:25 A time and cross fingers for asleep around 3:30.
If her nap is between 40min and an hour, should I increase the following A time next day?
I think you need to hold it at 3.5hr consistently for a few days before making any changes as she needs things to be consistent to get the sleep she needs and at this age, if she has 2x1.5hr naps in the day, you'll have a very short night, so you will probably find you need one of those naps to be ~40min anyway. She may do incredibly well on 2x1hr naps as my DS did at this age. Just if you keep the A times consistent then she can start to take the sleep she needs. Most important is to get that first A time consistent so we can get an idea of what may be causing those NWs.
If her nap was less than an hour, should I decrease her next A time? For example if her morning nap was only 40 minutes, should I give her say 3 hours 10 min of A time?
Well, this stuff is so variable between children that its hard to know what to advise. The general rule of thumb is to reduce A time by half the lost sleep but we don't really know what the 'normal' sleep time is for her, so I think just keep it at 3.5hr and be ready to resettle an OT waking from the nap rather than reducing the A time.
If I nurse her to sleep, will she be able to transition from sleep cycle to cycle? I just don't want to hear her cry anymore (at least for the next week and then start weaning her of nursing to sleep again).
Possibly. It is unfair on her to wean off feed to sleep and then feed to sleep again when she's likely in the normal regression that happens when you've weaned a prop. It will make the next time you try to wean longer and probably with more crying. Its really important to be consistent. I would comfort some other way rather than feeding to sleep so soon after sleep training.
If she wakes at night, can I assume that it is hunger? Or should I assume it is hunger if it was more than 4 hours?
I think by this age, its unlikely to be hunger every time unless there's 4-5hr between wakings or she's eating way too much solid food and missing out on milk calories in the day. Early on in the evening (up to 3hr after BT) is usually OT from the day (this is the most reliable, even with an inconsistent routine). Long happy wakings in the early hours of the morning is often because that first A time isn't long enough. These are rules of thumb but without a consistent daytime routine for her, its very hard to decipher any signs from sleep length or wakings.
My main suggestion is to keep to 3.5hr A consistently for at least 3-4 days so she can have some predictability and let her settle into the routine and take the sleep she needs. Your job here is to provide appropriate sleep opportunities and hers is to sleep. She can go to sleep, we know that. Your job now is to be consistent and let her body have a chance to take what it needs in terms of sleep.