Is there currently an answer for my specific need to develop a cross-platform installable web application?

I have very specific needs for an application that I'm developing. At first glance, it would seem that a Progressive Web App is the answer, but I'm having some doubts.

1. I want to offer a SaaS application to event marketers(the customers). The customer will create a page for an event on a subdomain of my app's root domain. The customer will display QR codes that link to their subdomain at the event that they are marketing.
2. The attendees of the event(the consumers) will scan the QR code, opening up a web page. The web page must immediately prompt the user to install the application to their homescreen, and quickly uninstall it in one to a few days at the most.

My limitations and concerns are:

1. It is important that the temporary nature of the app is emphasized. The app cannot be native as this would require an app store listing for each of the subdomains, which isn't feasible for a large number of short events.
2. Is there a way to get the "Add to Homescreen" prompt to show up immediately upon visiting the website? I have heard that it will only show up if the site is visited twice with more than 5 minutes in between visits.
3. The solution must be cross-platform, at least for iOS and Android. I'm not too concerned about accessing native device features. The only ones I could conceivably make use of are location services and notifications.

Does something exist at this point in time that would provide me with what I need?