I will note up front these rules are completely untested, and at least a little incomplete. I have drawn inspiration from 1PG and Fighting Fantasy.
Ranged combat is essentially unchanged, although only Luck contributes to adds.
A character must face off against a specified opponent. A character can only attack one character (or formation? See below,) but may be attacked by any number of characters.
A character in melee makes one combat roll in a round. This is applied offensively to the character being attacked, and defensively to all attacks.
Alice and Bob are attacking Eve, who is attacking Alice.
Alice takes 1 damage (16-15=1) and Eve takes 1 damage (17-16=1).
To combine attacks, each participating character must make an SR on IQ, Level (# participants - 1). This may be done in advance, providing the group can maintain formation. A combined group stays combined until forced apart or it disbands.
A group combines its HPT. If combined, characters in the group may take hits from another team member.
Alice and Bob are attacking Eve while combined. Eve is attacking Alice.
Eve takes 16 damage (15+17=32, 32-16=16). Alice takes 1 damage (16-15-1), which Bob may take.
You may attack at team as a whole, or just an individual in a team.
Combine all hits taken in a turn, then subtract armor.
At present, your melee example effectively has Eve attacking both Alice and Bob, since her full HPT is being used individually against them. If you had 5 people attacking Eve, she would be getting 5 times her HPT. I don't think this is workable as it stands. I'd be interested in seeing revisions though.