Why does DMARC operate on the From-address, and not the envelope sender (Return-Path)?
Think of SPF and DKIM as ways to validate the mail path, and think of DMARC as an extension that also validates the message sender. Think of this as delivering a FedEx letter. It's easy to validate where the envelope was shipped from, and that the courier was legitimate, but it doesn't provide a way to prove that the letter inside the envelope is really from the person whose name is printed on it.
Your webserver is a valid SMTP server for mywebserver.com and that your Sender address is legitimate, but that's not enough for other servers to trust that you have permission to send as [email protected] . How does GMail know that your server hasn't been hacked or otherwise used for malicious intent? Gmail's servers aren't going to blindly trust you to send mail as one of their users -- unless maybe you are hosted by them, and then you'd probably have trouble sending to Yahoo.
To address your first part of the question, yes, it's very likely that this is why GMail is categorizing it as spam. The oldest forms of spam center around spoofing the "From" address. This is what most users see when they get a message, and is the primary field they want to trust. When a message from a legitimate mail server is sent using a From address that doesn't belong to that mail server, it's still a red flag.
As you mentioned, DMARC operates on the From address as part of the specification. Granted, it makes it harder to write web apps that send on someone's behalf, but that's sort of the point. As to why they do it - well, that's up to the designers of the specification, but it's a trade-off. They are taking the high road and making a system that works very well if you stay within that limitation. Perhaps future mechanisms will find a way around this.
The unfortunate solution is to only use addresses that you have control of. To address your third question, sign your messages with your domain name, and mention in the body that it was sent on behalf of [email protected]. Otherwise you will have to request that your recipients add the address to their whitelist. It's not much fun for a legitimate web app developer, but it will protect the sanctity of the recipient's inbox. You might have luck using the Reply-To header with the web user's email address.
There is a discussion of this limitation on this DMARC thread.
In the mean time, you can try to make sure that your server isn't blacklisted on any RBLs. It could be that you can fail DMARC but still get through some spam filters if you have good enough reputation... but I wouldn't rely on it.
1) yes, likely the dmarc failure will cause gmail to junk your mails
2) also would be interested in an answer for this
3) I would (and we do) use the reply-to field for the customer address, our mails look like this:
from: [email protected]
subject: contact form
reply to: [email protected]
Hope this helps