hej hej ...
here are the answers you need...
1) yes... you have to plan it on your own, as InterRail is just a normal train ticket. To arrive at the point where your InterRail ticket starts or is valid, you have to find your own way. I think the cheapest way will be to fly by Ryanair or Easyjet from Germany to the UK.
2) yes, you can buy the Youth 2nd Class ticket as you are 20 years old.
3) if you buy the InterRail Flexi Ticket for the UK valid for 8 days, you have 8 travel-days within one month.This means: you can chose 8 days for free within 30 days you want to travel. You are flexible like the name of the ticket.
And the best: if you travel by night-train, which is leaving after 19:00h in the evening, fill in your ticket the date of the following day and you can travel by this night-train and the full next day until midnight!
A travel day is always from 0:00h in the morning until 24:00 in the night.
4) when using a ferry between England and Northern Ireland, you have to pay extra. but you will get reductions. have a look on rail.cc/en/interrail-ferry/united-kingdom/uk
... it is not very expensive to use one of these ferries.
5) yes, you can buy an InterRail Felxi Ticket UK and an InterRail Felxi Ticket Rep. of Ireland ... no problem to combine them. but remember, when you use a train going from Northern Ireland (belongs to the UK ticket) to the Rep. of Ireland, it counts as two travel days - on on the UK-ticket and on of the Rep.Ireland-ticket.
6) As the InterRail ticket is a train ticket, you can use it only for traveling by train. in some countries you can use buses, but these a special buses operated by train companies... this is not the case in the UK+Rep.Ireland.I hope this answers will help you !!
have a lot of fun and buy Interrail here: rail.shop/interrail