Numerals

I have some questions about how numerals work.

I've started learning only 0-10 so far, and I've got really numerous questions. Also, I'm a little confused with the difference between the standard and the spoken languages

1. Is "sero" used in Welsh and can it be pronounced with [z]?

2. Does "un" cause limited soft mutation in sg. f. noun?

3. Do "dau" and "dwy" cause soft mutation for all the nouns?

4. What is the difference between "ail" and "eilfed" and do they both cause soft mutation?

5. Is "cynta' " used in the spoken language

6. What happens with "tri" and "chwe" in the spoken language (I mean do they cause aspirate mutation)?

7. What the difference between "pum" and "pump"; "chwe" and "chwech"?

8. Does "seithfed" cause any mutation?

9. What is "deng"?

10. Do any other numerals cause any mutation, and I've missed something?

11. Does "cyntaf" follow a noun?

12. Constructions with numerals are either numeral + singular noun or numeral + o + plural noun, right?

I'm extremely thankful to everyone who will respond to my questions