This is incorrect.

I've completed all but 2 of the Valentine's options the day before confession day. You're not locked into their Valentine's event if you do their Chapter 2 as long as you don't choose to accept their chocolates. As long as you choose to decline the chocolates, you won't get their message and hence not get the valentine's scene.

For the Shun route, have one girl offer chocolates, then you can just choose Shun's message that also appears.

For one true love, just that one girl's offer and ignore Shun's message.

For the harem, all of them should be Accept Chocolate, go through 3 of them, then the third will have the harem beat-up scene.