Yes, you default to the variable rate if you take no action.
No fees if your fixed term is up
Yes, this is exactly what you should be doing. You are essentially getting AIP from another lender will will be valid for ~6 months so you should get everything organised in advance of the fixed term ending
Yes, you will need a solicitor for a new mortgage so it will be in the region of €1200-1600.
Just be aware you can switch mid term with Avant if their own rates drop. You just need to check if a break fee applies
The costs to you are a potential break fee, the solicitors fee and a valuation. You need to balance this with the interest rate reduction and/or any cash back you may receive when switching