-Do different skills have a hidden multiplier for damage or are skills with same description always identical in damage?

would like to know this too

-For example if two characters with same stats have a skill which causes ninjutsu damage to 3 opponents with no special combo or other factors described, is their average damage same?

Its probably same base damage (or RNG from base damage) and then reduced by resistance, defense etc

-Some skills will say stuff like "Causes heavy ninjutsu damage" so that means it's better than normal ninjutsu technique?

- Do characters have built-in factors such as someone has more ninjutsu than others when you recruit them? Do some gain more ninjutsu than others as they level? Is their damage etc. always in relation to the shown stats or are there hidden multipliers for different characters that affect these outcomes?

There are different hidden tiers of ninjas (blue, purple, yellow, red), im guessing they have different stat gains but it does'nt seem to make much difference in practice

- How much more damage will a skill with ninjutsu + taijutsu cause compared to only ninjutsu or only taijutsu? Assuming of course the character has same amount of both

I saw a thread about the damage formula here on forum and it says that outgoing damage = 50% NIN + 50% TAI for such ninjas, but in practice ninjas with hybrid damage seem to do much more damage than ninjas with pure damage, so it might not be true

- How does combo affect damage? Say I have a skill "Cause ninjutsu damage to 1 target" and "Cause ninjutsu damage to 1 target and 5 combo" will the latter for example cause 5x the damage on average? Or will it do 5 hits with only 20% damage each?

If you check the damage numbers in game it shows that each hit does a fraction of the total damage

- Will damage go up as I get more combo? Will the 20th strike be more powerful than the first?

Definitely not, they are all same

There is a kakashi NPC in south konoha, you can fight him to test different teams and measure damage