1. Each standing or routed hexagon unit is worth 1 strength, regardless of health or rank.
2. In your example, the game would end immediately BEFORE the summer season card is drawn but only if he has 6 or more dragon runes. If he does not, he takes his victory card back and the summer season card is drawn.
3. You can bring more than 8 units to an activated enemy or neutral area, as long as your intention is to fight. (You have no choice if you move into enemy territory, but you cannot attempt diplomacy if you move more than 8 units into neutral territory.) If you lose the battle, all of your units will retreat. If you win the battle, units in excess of 8 will retreat.
4. Unallied neutral units retreat to adjacent neutral or empty areas. If they cannot, they are destroyed. The player to your left chooses where to retreat them. Allied neutral units retreat along with your units.
I find the rules for retreating clear, but not always obvious, so I've summarized them below.
As a reminder, a controlled area contains at least one of a player's units or a stronghold, and may also contain neutral units (allies). A controlled area is friendly if it is controlled by you, and enemy if controlled by another player.
An uncontrolled area contains no player units. If it contains neutral units only, it is a neutral area. If it contains no units, it is empty. Heroes by themselves do not affect the alignment of an area. Heroes may be present in an uncontrolled area, whether neutral or empty, and it would still be considered neutral or empty and therefore uncontrolled. This includes commanders.
Player-controlled units, which includes allied neutral units, retreat for 3 reasons (in addition to card effects): A) You lose a battle - all surviving units retreat. B) You win a battle, but your survivors exceed 8 - the excess must retreat. C) You fail a diplomacy test against unallied neutral units, and you choose to retreat rather than fight - or you are forced to retreat, if you played the Mobilize order and you had already fought a battle.
Retreat to adjacent friendly areas (do not exceed 8 total units per area) until no such areas are left.
Then, retreat to adjacent empty areas (do not exceed 8 total units per area) until no such areas are left.
Then, destroy the rest.
If you lose a battle and all your survivors are allied neutral units, they still remain allied. You retreat them to adjacent friendly areas. If you cannot, you retreat them to adjacent empty areas and then they break their allegiance. (As an aside, if you WIN a battle but all your survivors are neutral, they immediately break their allegiance.)
Unallied neutral units retreat for 1 reason (in addition to card effects): the middle result is drawn on a diplomacy test. They follow this pattern.
Retreat to adjacent uncontrolled areas (neutral or empty). The player to to the LEFT of the active player decides the areas.
If no such areas exist, destroy the rest.
Cards which require retreats will follow the rules for player-controlled or unallied neutral units as appropriate.