(1) Can I expect that a reticulum based communication....
Not really. Most are used to centralized server based comms with push services. If you have all set up properly and are using a properly set up and reliable interface for transport, you can expect pretty much very fast and stable comms. If you go over LoRa and for example use a TCP interface server somewhere half-way around the world, that is not capable or set up properly, you might expect delays and/or messages not going trough. This is why it is important to properly set up local resources like TCP Backbones, LoRa-TCP gateways on good locations, local propagation nodes, etc. You cannot expect to buy a cheap LoRa node or use someone else's TCP Backbone, a random propagation node and have reliable comms. This is what owning your means of communication means - actively developing them to be reliable, stable and lasting. In my case I can rely on all these locally, so my messaging, with tiny exceptions, is very fast and smooth.
(2) So far the feature "messages are delayed, not dismissed...
Again - just select a reliable propagation node. I cannot judge for others and I have no idea where you are, but I can suggest you try rns.sofia prop, because I can say it is very much stable and absolutely properly configured - 1e96a12e2ac59bfebd604a6e3cb95a68 Sometimes LXMD dies on me, but i catch it pretty fast and restart it. For now I have no active means to automate this.
(3) Is using a lora pager like device in a small city a realistic alternative...
Well it depends heavily on many factors. IF we take for granted you have already setup a proper backend, TCP, prop, etc., the radio aspect depends heavily on propagation conditions in your area, laws, gear, placement of nodes, setup of nodes, etc. Small town is far from good description of where you want to deploy, what you want to deploy. Reticulum is not an easy drop-in solution for anything, it's a tool you have to learn to use, deploy, maintain. In my case it took mor than 7 months to get to this - rns.sofia network and I still have to put up the LoRa network, so any meaningful coverage can allow for me to go over LoRa fully and dismiss TCP. Yes, I am in a 2.5 million deep city and running 433MHz, but still.
(4) When using a smartphone, is it realistic that the "app" (like sideband) doesn't use too much of the battery...
You are no more the end user to a service. YOU ARE the network. There is no PUSH, no servers doing the heavy lifting. YOU ARE the endpoint. You have to listen to announces, resolve paths actively, wait for messages. This takes battery. And if you go for the ultimate goal of SHARING your hardware by enabling Transport mode, so you can retranslate other's messages, you will drain battery even faster.
(5) What is the easiest way to support reticulum for somebody who is...
https://liberapay.com/~1880498/