I don't think that projector has enough lumens, that is bright enough, to to do what you want to do for "Decent visibility of movie on the screen in late afternoon/early evening hours". Projector Central's calculator gives the LG PF1000U 17fL with a 100" diagonal 16:9 screen with 5% ambient light; in other words an almost blacked out room.
The LG PF1000U projector is rated at 1000 Lumens and has an LED light source. Most peoples practical experience with the current 1000 lumen LED/DLP light engines is more like 500-700 Lumens after calibration. I would suggest that you will need 3X the 1000 capacity or 3000 Lumins minimum to do what you want to do.
The "Visible Vitrum RPS 9 Plus" looks interesting and should yield good results, but again, I think you're going to need more Lumens than the LG PF1000U can deliver for late afternoon viewing. On Visible Vitrum's home page, they reference having "our RPS 9 Plus coating, a 3000 lumens projector". https://www.visiblevitrum.com/
This video of Visible Vitrum rear projection in daylight, is showing what looks like an Epson projector that I would guess to be in the 3000 Lumen range:
There is a company called ICE AV Global that sells a similar product as Visible Vitrum for front projection and they too recommend your projector be something around 3000 Lumens for out doors. Here's a link:
Then there is a company called UGO that sells a 3M film product that can be attached to glass or plexiglas for rear projection that would also work for you I think, but again, they recommend something around 3000 Lumins as a starting point. Again, here's a link for you: https://www.ugo-media.eu/materials/r...ection-screen/
As an aside, I just returned from Bilbao, Spain where I attended an exhibit titled "SIGH" by the artist/photographer SAM TAYLOR-JOHNSON at the Guggenheim Museum. Taylor-Johnson's exhibit used multiple digital projectors to rear project on multiple floating screens. The floating screens were made from a plexiglas/acrylic material with rear projection material (something like Dazian) stretched on one side where the image was projected. This allowed the guest to walk around the room and see the multiple screens from any position, front or rear, and was very effective. Here's a link to a video for the exhibit/show:
Taylor-Johnson also directed the 2009 film "Nowhere Boy", based on the childhood experiences of the Beatle songwriter and singer John Lennon.
This screen was very cleaver and I think low cost to make: Here's a link to Dazian screen material. Something like TRAPEZE® PLUS was the material stretched and attached to the plexi. https://www.dazian.com/fabrics/projection-lighting.aspx
As for Audio, I'm not sure I understand the question. Maybe you're concerned about sound levels and the neighbors? If that's the case, it's going to be difficult to keep the sound levels down and not disturb the neighbors with a surround sound set-up. I would maybe suggest you use wireless speakers and run your AV receiver in mono. That way you could place a speaker very near peoples seating positions in multiple locations around the pool and enjoy the picture "Drive Inn Movie style.
Sounds like a fun project. I think what you want to do is doable but you're going to need a projector with more Lumen capacity to work. Please up-date your post as you complete your project. I'm interested to see how it turns out.