theDIBB - DLP upgrade to annual pass help

Hi all
As many, we have had WDW cancelled, then Lanzarote so have booked DLP for 4 nights in 12 days time as a pick me up.
There is me, DH and 5 kids (although 3 counted as adults in Disney price terms)

We would like to take my in-laws at Xmas as and are considering a half term too.
Flights are very reasonable - cheaper than us going by tunnel (too many hours driving) and much cheaper than Eurostar I’m the holidays (I work in a school!)

We are looking at the infinity passes and hope for some clarity from
The DLP wise... ;-)
1) we have 4 day tickets that cost £1472 - would this be deducted off the annual pass price? I work this out to be £2965 on a 1.06 exchange rate. (That’s 7 infinity passes due to school holidays)

2) how does monthly payments work?
Is it easy to sort?

3) how do you go about buying discounted tickets eg: if we take my in-laws?

That’s it I think 😂

Thanks in advance
lou