Actually, TB, I found the "when" part of the drapes-opening question in Ceasar's Dealer's theory to be rather interesting.

He (or she) seemed to naturally assume that they were opened while Kay was asleep, which could makes sense since Kay presumably would have noticed that they were open when she started to get undressed.

Like you, I had always assumed that they were opened when the room was empty.

But then, I realized that they had to have been opened prior to her entering the room, because...

Well, let me explain it this way:

There are six possible scenarios for the drapes opening:

1) Michael enters room first, drapes are already open
2) Michael enters room first, drapes are not yet open
3) Kay enters the room first, drapes are already open
4) Kay enters the room first, drapes are not yet open (Ceasar's theory)
5) They enter together, drapes are already open
6) They enter together, drapes are not yet open

This raises questions:

Was it assumed that they would get a clear shot at Michael when he went to close them, or simply get a clear shot because they were open.

Let's assume the latter, since they had no way of knowing that he would go to close them

Since the killers could not have known who would enter the room first,

#1 is perfect for them, since they get a shot at Michael immediately

#2 is bad for them, since if Michael is in bed before the drapes are open and the lights are out, how do they get a clear shot at him?

#3 isn't great for them, since Kay may have noticed they were open, and closed them before Michael got there.

#4 which is Ceasar's theory, is risky because the drapes-opener might've awakened Kay when entering the room

#5 is good for the shooters (same reasons as #1)

#6 is bad also, for the (same reasons as #4, except they risk waking Michael)

How did shooters know who would enter first, or if they would both enter together?

In fact, depending on the drapes being open for the assassins get a clear shot is quite problematic.

But clearly, the least problematic scenarios involve the drapes being opened first, regardless of who enters the room first, even though if Kay enters first (#3) that could ruin the whole plan if she notices and closes the drapes before Michael gets there.

You have to think that the drapes were opened while the room was empty, since scenarios #1 & #5 are by far the best from the shooters POV.

For Ceasar's theory to be workable, the shooters would have had to know that
1- Kay would enter the room and fall asleep before Michael got there, and
2- Assume that Michael would turn the lights on when he walked in, risking awakening Kay. Wouldn't it have been more likely that he would simply undress in the dark and slip into bed?

The more I think about it, the more ridiculous the whole drapes thing becomeas all together.

Picture the discussion....

Roth: How are we going to do this?

Ola: Someone will sneak into the bedroom when it's empty and open the drapes. Then when Michael walks in we open fire.

Roth: That works if Michael walks in alone, or if he and Kay walk in together. But what happens if Kay walks in first and sees the drapes open and closes them before Michael gets there?

Ola: Well, we could hope that Kay enters first, and then when she's sleeping, sneak in and open the drapes. Then when Michael gets there, we open fire (Ceasar's heory).

Roth: That idea is even worse. Suppose Kay wakes up? Suppose Michael walks in while the drapes opener is still there? Since Kay will be sleeping, Michael probably won't turn the lights on and we won't even know he entered the room.

So after close analysis, the whole "Drapes Idea" stinks.

They had to have been opened when the room was empty, and even then, they had to depend on Michae entering first, or Kay and Michael entering together.

Altho Kay apparently failed to notice that they were open while getting indressed (and probably gave the shooters a free show), it's hard to believe that an old shrewdie like Roth depended on that happening.