Because este is being used as a noun here, meaning the east.
In Spanish, compass directions often take the definite article when they refer to a region or area:
So El este está soleado hoy means that the eastern area is sunny today.
It uses está because soleado describes a condition/state, like the weather today, not a permanent identity.
Spanish usually uses estar for weather conditions in a place at a given time:
Using es soleado would usually suggest something more like an inherent characteristic, which is not the normal way to talk about today’s weather.
Because it agrees with el este, which is a masculine noun.
In Spanish, adjectives usually match the noun in gender and number:
So here, soleado is masculine singular because el este is masculine singular.
Putting hoy at the end is very natural in Spanish. It works like today in English: it can move around depending on emphasis.
All of these are possible:
The version with hoy at the end sounds neutral and natural.
Yes, you could, and it can sound more natural in some contexts.
Compare:
The first treats the east as the subject.
The second uses a location phrase, which is often very common in weather reports.
You might also hear:
The accent mark shows both pronunciation and meaning.
So the accent is important because it distinguishes two different words.
Usually, no. Compass directions are normally written in lowercase in Spanish:
So El este está soleado hoy is the normal spelling.
You might see capitalization only in special cases, such as part of a proper name, title, map label, or heading.