There's quite a few bits in there, so in no particular order...
First off no network guarantees 100% national indoor coverage, but EE do offer WiFi-calling as an excellent mitigation - this allows full use of your phone for calls & texts without needing mobile coverage. Every network has good & bad coverage areas, and anyone who suggests otherwise is telling porkies!
The "best network" claims are based on national benchmarking & comparison tests, and across multiple testers & methodologies, EE does consistently perform well with other particular networks regularly performing badly with their own issues - in some cases, a network can be known for often having good indicated coverage but an inability to use mobile data due to lack of capacity or issues with the 4G/2G fallback.
As long as your individual phone has VoLTE support to allow use of the low-band 800Mhz spectrum, then variations between specific devices should be relatively minor down to the radio-chipsets and some software differences.
The coverage maps are generally a reasonable estimate of service, but are never a guarantee and don't always have the granular detail to show small local variations or allow for geo-physical factors.
