On the personal allowance question - it’s the £12,500 state pension that counts towards the personal allowance tax free element.
The state pension is paid in full, without tax being deducted and HMRC adjust your tax code on your remaining income.
You are probably aware, but just in case - the first taxable withdrawal from a SIPP is usually taxed at 40% initially and you can claim back the difference from HMRC. So your initial payment would be more like £35,000 with £7,500 to claim back to get you back to the full £42,500
In my case it took about 3 months to get the overpayment back from HMRC, but then HMRC sent a tax code to ii for me and it was sorted at 20% for subsequent withdrawals.