No you don’t, that was 6 paragraphs. 
No, there is no user control over individual entity retention. There is one purge setting for all entities.
Long term statistics are never deleted.
Short term statistics and state data are purged after your keep days setting.
You could set up your recorder to purge state and short term stats data after one day and rely on LTS for hourly average, minimum and maximum (or total and sum for non total state class) for looking at data further back in time. If you are happy to drop to that lover hourly that resolution.
It depends on the integration but mostly no. There is also this How to set a custom scan_interval I would only do it for very chatty entities.
Also this add-on was released the other day. It makes it easy to see what the most talkative entities in your database are: Dbstats - Addon for database usage analytics