This all depends if you are going for a feel of hopelessness and horror, or if you would like your player to feel they always have a small chance of success.
Having just grabbed the pdf and read through it I would suggest:
"If a character is alone, can she roll for a risk type other than her own? Like a "clever" risk for a "dangerous" survivor. If yes, that means only dice taken from the supply pool?"
That you can always take dice from the supply pool 'At any time during the game, a player can draw dice from the Supplies Pool' (p7), but if you wished to instigate a house rule to allow for a small chance success of: any survivor except the helpless survivor gets to roll one dice for any risk type not there own; that would seem reasonable and not excluded by the rules.
"If there are three characters (e.g. Dangerous, Sneaky and Fast) and a “Clever” action is needed: may one character attempt the action using two dice? (No archetype dice and two bonus dice from the helping characters.)"
I would say you get the 2 dice + one for house rule if you go that route as: '... if you are with other people, you can roll a number of bonus dice equal to the number of people with you.' (p6) This does not seem to be exclusively if you are rolling for your risk type as if:
' ...are out getting groceries at the local food shop, trying to sneak by the zombies, you roll a number of dice appropriate to your Survivor Type plus a number of dice equal to the number of people with you.'(p6).
So if your survivor type gave you 0 or 1 die then you would always get the +2 for the 2 people with you.
"If a character is aided by others in the group (thus getting bonus dice), do those other survivors still get to roll for themselves?"
I would say it depends on the level of despair you want, if you are after utter despair I'd suggest NO, as if the clever guy can't figure it out no-one else stands a chance. But, if you want a little less despair you could let a re-roll for the clever guy the next day having slept on it, but with a dice pool penalty, or let someone else try it the next day but with the 0/1 + any help, may be with a penalty.
The penalty could be due to the fact the clever guy failed because there was some damage; so the Fast survivor can role with help, but at a penalty lets say -2 dice. He rolls and passes - the fast survivor hits it in a fit of rage knocking something into position after fighting with it and it splutters into life.
The short answer is do you want to have hopeless despair, fear and horror, or do you want to give people a small hope of survival, a decision for you and your party. As always do what makes sense for you and the people you play with.