controversial question: discipline & spanking

OK people. We're in a stage of parenting where we're trying to make some solid decisions about some controversial issues. I'm genuinely curious about your thoughts regarding this so please feel free to share them. I haven't made up my own mind on the matters yet, and promise not to judge any comments/opinions made here.

We've tackled children & nudity, now onto discipline & spanking...

Lily's definitely entering that stage that some like to call "the terrible twos". She's still 6 months away, but she is a lot more defiant (and independent, which isn't necessarily a bad thing) than ever before, and we think it's only a taste of things to come. 

She's a bit slow on the uptake when we ask her to sit in the bath (the girl LOVES to stand...yes in the cold air instead of the warm bath) and sit in her rocking chair. These are two really major areas for disobedience for her right now. They seem trivial, but they're important to us for two reasons: they're rules in place for her safety (standing/walking in the bath can and has lead to wipe outs, and standing on a rocking chair is...well...obviously dangerous) and they're rules she understands and consistently disobeys.

I hesitated to write the bath example, because yesterday she only had to be told around 5 times to sit, whereas normally it's more like 30+, but I'm not sure it's because she's learning and obeying, but more so because she was tired and in a good mood.

So far, my method has been to give her a time out (60 seconds for a "minor offense" like not listening and 2 minutes for a "major offense" like hitting Oli or Brad and I), which worked at the beginning, but her independent spirit has now made her perfectly happy in a time out. She always finds something to play with in the bathroom that contains zero toys... so it's not so effective anymore. And from the get go, we've used a stern, even voice when rebuking Lily, which also doesn't seem to do much.

I should also note that as the day progresses and she becomes more tired for her 6pm bedtime, she becomes more disobedient. I don't think I've ever given a time out before 3pm. And I probably give the most between 4pm-5pm.

Which begs the questions: what forms of discipline are effective for 18 month olds? or 2 year olds? do your kids have a "difficult hour(s)" like Lily does around 4pm? do you give greater grace during the "difficult hour(s)? what do you do with your kids? what do you plan to do with your kids? what was the case when you were growing up?

Here's where it gets tricky. I'm not opposed to spanking, especially in the two mentioned cases (disobedience involving danger to the child and consistent, repeated disobedience), but for a couple of reasons I don't think we'll use the method. 

Firstly, I don't know how hard to spank. When Lily would consistently throw food off her highchair, I took the suggestion of many parents and tapped/slapped her hands. I thought I was doing it hard enough, but Lily didn't flinch. And she's not good at ignoring pain. So I tried harder. Still nothing. Then it occurred to me: I'm certain I'm tapping/slapping hard, but do I just continue until I get the desired results? That seems like a sordid slippery slope. I don't trust myself to spank/tap/slap at the appropriate level of intensity or strength.

But secondly, and most importantly, we don't want to start a practice with Lily and Oli that we can't continue with our adopted child(ren). And you absolutely can't use spanking as a form of discipline with adopted children. Whether the child has been subjected to physical abuse or not, part of the adoption process in Quebec is signing and swearing that you will not discipline with spanking. Based on the history of violence that many children have who are adopted, I support this rule 100%. But it does make disciplining my first two children rather difficult. I don't want to be inconsistent with standards and discipline, and I don't want to get comfortable with one method, only to be back at square one in a couple of years, looking for another.

Would love to hear your thoughts and experiences!