Well, if we are keen on having a squad share the HP pool, and not lose any soldiers ...
These do not need to be combined. It is perfectly fine to have a squad "lose" individual models as it takes damage, but to automatically regain those models when it heals back to full strength. This is the design I would favor.
So in once sense the hp pool is shared (the unit will heal back to full strength and numbers as long as it has even one hp left) but in another it is not shared.
We also don't need to change the movement/combat speed or anything like that; we don't need to model the individual "attacks" separately.
Its just, when a unit with 3 guys and attack 5 makes a single attack against an enemy, the game processes this as 3 separate attacks of strength each against the entire defense value of the defender.
All from a single right-click attack command.
[Another advantage of this; by a central limit theorem, the distribution of the result of 3 attacks of 1-5 will be much more weighted towards the mean than will the result of a single attack of 1-15.
So adopting this kind of system will also help make combat results more predictable.]