The offical borderpoint is “Frankfurt(Oder) Grenze” it´s behind Frankfurt(Oder) HBF so you have to book a ticket till the Borderpoint or to Rzepin :)
I usually book a saverfare from Berlin to Rzepin (fares from 18€) as the ticket to Borderpoint can only be issued at a ticket office

Note you need a reservation within Poland with Interrail (price for domestic reservations is 1Zloty )

I book usually first the ticket Berlin - Rzepin(here you get a reservation for free)  and then reserve afterwards the leg Rzepin → Poznan/Warsaw/Krakow/Gdynia online at Intercity.pl and select the same seat as from Berlin :D


Finally check if it worths to book a saverfare to your first destination in Poland as you get saverfares from 29€ to the major Polish cities and use then the one countrypass for trips within Poland.
 

No/nein. The railways have invented -many, many years ago, a solution that uses the exact borders: think of them as stations without any platforms or ticket-counters, its called Grenzpunkte/punkt granica. A few KM beyond Fra/Od, on he bridge over the Oder/Odra. Untill that point you pay DB-Tariff, from there the much lower PL/PKP prices. INTernational fares are simply the sum of each side added together.

DB can sell you those tickets only at counter (that is, if they still want to do, I guess they only do together with you buying the pass on paper from them). 

If you have more time as money, you could go via Kosztrzyn/oder (a bit north) or even Szczecin-as to there BVB/VBB or DB fares apply- even less as 13€ from B.

If you come from further in DE, it might be that advance fares (Super/Sparpreise) to Rzepin work out even cheaper, but then of course you are tied into those days/trains.

Also note that PKP (=Fernverkehr In PL) or also PolRegio (Nahverkehr) offer much cheaper passes, only valid inside PL on their own trains, PKP for midweek, PR for 3 days/month (less as 20€) or a whole weekend.

A 1-country pass for PL (also global passes) are -quite different from DE, NOT valid in each and every train. There are now many local regional companies (f.e. KM=Mazowieckie around WAW) that do NOT take them. They all have their own passes/offers, But PKP serves-not that frequent and sometimes with quite old trains- all major towns. ALL their trains have mandatory REServation-you get that for free with pass at their own counters (IN PL there may be different counters for every comp. in 1 station). Brush up your Polish!

It MAY even be, I am not sure, that if you take the very few PR trains from FRA/Od your pass is already valid. THis is f.e. the case from Görlitz on KD=Dolnoslaskie, but this is of course much slower and more cumbersome as the direct EC B-WAW.