1) canned or frozen food is fine - just check for salt/sugar content. tinned/frozen/dried fruit/veg all count towards a fruit/veg portion
2)the fruit doesnt count in a yoghurt as it wont contain enough of it - the yoghurt counts as a dairy portion.
3) it depends on how you feel about it. we were never particularly cautious with dd or ds (they had peanut butter by 1 year and had tried eggs, etc)
4) I would avoid adding salt to foods but you could use herbs, garlic, etc to add flavour. Up to 1 year they should only have a maximum of 1g of salt per day and if you add up the naturally occuring salt plus any in processed foods such as bread, cheese, etc then it is easy to get to that figure without adding to it yourself