This is not an HMRC question and I am not sure that you have quite finished the "hard work" you mention.
RiA is the German tax office and, unsurprisingly, deals only with tax, not claims for state pensions - that is dealt with by the Deutsche Rentenversicherung (DRV) Bund in Berlin, BUT if you are UK resident you claim that pension via DWP International Pension Centre in the UK. They send you a form CFN901, which, once returned, enables them to pass your claim and UK NI record to Germany. If you have at least 35 years of UK and German social security records, then you can claim your German pension at 63, albeit reduced by 0.3% for each month it is taken before normal pension age. All a question of maths... You might also find that your German pension will be higher than you think because of how their pension system interacts with the EU regulations on pensions but that depends on your work/non-work history and is for DRV to explain.
I fear also, that you have misconstrued the German tax rules. The starting point is that you are subject to limited liability taxation as a non-resident. This means that 100% of your income arising in Germany is taxed without recourse to any allowances or reliefs, other than a Werbungskostenpauschal für Rentner of 102€, i.e. the Grundfreibetrag is irrelevant as you do not qualify to get it. The 102€ will be deducted automatically from the taxable element of your pension when your tax assessment is issued each year.
Deferring your UK pension is, in my view, generally not a good idea as you forego 100% for the deferral period in return for 1% more pension per 9 weeks of deferral going forward. Again, a question of maths but payback is the best part of 17 years...
As far as German tax is concerned, there is a special rule which says that if at least 90% of your income is German-source, OR your income that is not subject to German taxation is less than the Grundfreibetrag, then you can elect to be treated as if you were tax resident in Germany (a tax fiction) and thus gain access to all allowances and reliefs. However, this is an annual election based on your income in each German tax year, which is of course January to December, not April to April. Also, all of your UK income would then feed into the rate at which you pay tax on your German-source income, even though it would not be taxed, and it uses up the Grundfreibetrag in the process, so any tax saving can be tiny. You only need to prove your UK income if you are making this election because it is otherwise of no interest whatsoever to the German tax authority. So, whatever it is you think you may gain by deferring your UK pension to somehow gain an advantage looks to me like a complicated and potentially fruitless nightmare.
Subject to the aforementioned election, Germany has no interest in your UK income and simply takes the taxable element of your pension, deducts 102€ and sticks the balance into their tax calculator. There are plenty of German tax websites that give you tax calculators but as a non-resident, you have to add the Grundfreibetrag to the taxable part of your pension for them to work - the same goes for the official tax office calculator on their website. This is because the German tax tables assume you have access to the Grundfreibetrag, even if you don't.
I would respectfully suggest you have another read of the RiA website (the German version is more comprehensive but the English version gets better every year) to understand the tax issues you will be facing. I would also suggest having a chat with the International Pension Centre to move you forward in terms of the hard work...
As to your bonus question, the answer is "it depends". A Euro account is good if you want to spend some of the money in Euros, or want to manage when you convert it to pounds but neither has a bearing on your tax.
