Conditional redirect based on password entered?

Site URL: http://gregandaimeegettingmarried.com

For a wedding website (currently on a Brine family template), we want to greet our invitees with a welcome/password/splash page type screen -- and then, depending on their entry into the password field, direct them to one of two pages:  (1) an index page with livestream info or (2) a different index page with various details for attending in-person. 

I know this is a little complex for a wedding website, but good etiquette (and some family dynamics) mandate not sharing info about coming in person with people who aren't invited to attend in person (some people will only be invited to the livestream) -- and to boot, the people with only the "lesser" livestream invitation not being tipped off that they're "missing out" on attending in person.

The site isn't live yet (currently only a cover page is up), but after some intense Googling and searching the forums here, I've come to the conclusion that it must be one of the following:

• Custom coding is necessary that somehow impacts the lock screen
• Maybe I can somehow do this with custom coding on a form redirect, and use the page with the form as my homepage/splash page?
• This might be possible with an upgrade to the Squarespace "membership" package, or otherwise using a third-party membership package like Memberspace
• It's just not possible with Squarespace

... I'm just not sure which of the above it is.

Unless I can figure this out, I may need to double my costs (and workload) and have two different websites with two distinct URLs printed on invitations, depending on what various guests are invited to. I would REALLY love to avoid that.

I'm in communications for work and dabble in CSS, HTML, code injections, etc, but am not fluent in anything beyond that (though I can follow written instructions and video tutorials with the best of them!)

Any/all help or insight is very, very much welcome and appreciated! Thank you!

Hi. Have you found the solution yet?