1. I didn't get that impression at all. Either way, Helena would have hardly had any time to prepare, as it wasn't a big, premeditated plan. If it is Helena, she made the decision only the day prior, if not as late as that morning.
2. She wasn't thinking about it being night. That, and super rich Helena would absolutely have grown up with a gardener, think gardeners are common, possibly had ones that even worked at night, etc.
3. I don't quite follow what you're saying, but ultimately it doesn't matter because I will bet you any amount of money it's Helena down there, whatever logic be damned. It's what's best dramatically for the story and the writers aren't going to pass that up. Not to mention, Helena sees in Mark a freedom and a spark she doesn't have in her own life. That was the whole point of her watching the kiss on the security footage over and over, and then creepily watching him arrive to work, ultimately spurring her to make the decision to go rogue and pose as Helly.