Tenancy not registered with RTB

Thank you. From this it would appear once it is proved tenant no. 1 was in situ from 2010, the 'tenancy', such as it is (>8 years) would require notice to be given in termination of 224 days.
 
Personally i would not be signing anything that does not reflect the true situation and make that clear.
You say the rent was slightly less than market value, well paying €500 a month for two domestic bills is €6k per year, our modest 3 bed costs only €2,300 in those bills for the year so I've no doubt you would have been making up the difference in the slighly less than market value.
It would appear to me that the owner has been having their cake and eating it.
Will you have a home once the property is sold?
It may well have suited you to go with the flow while you were in the house but you won't have that particular roof over your head for much longer so I would do right by yourself.
 
You cannot demand a tenant sign away their entitlements. If there was rent to be paid, it was a tenancy, however "informal" the owner imagined it to be. They have accepted rent every month for 14 years and need to behave with some professionalism regarding this.
 
  1. Will signing the documentation as requested impact on the tenant’s rights as a 14 year in-situ tenant (e.g. notice period, issues regarding recent documented tenancy vs 14-year undocumented tenancy etc.)?
No.
  1. Can the two family members who moved in during 2020 be a licensee of the house owner?
Yes, probably
  1. Would potential RTB advocacy be based on the signed documents (if signed) vs no signed records (there may be text messages regarding collection of rent, servicing of gas boiler etc.)?
No, absolutely not. Rentals were run this way for years. Its up to the landlord to be professional, not the tenant. If he/she chose to not put things in writing that's not the fault of the tenant.
any thoughts as to my queries? please and thanks....
This is just my opinion, and to add the auctioneer doing the sale is only interested in that. They should not be relied upon for legal advice. The landlord at very least should give proper RTB notice period and a serve it according to the law, treating it as a normal tenancy and not just some kind of gentleman's agreement.