EE's from 1992 pay the higher of two different calculations:
guaranteed rates, which were 6% for 12 years then 4% at least through the current extension period which as I read it probably lasts until maturity in 2022
market-based rates, which aren't known for the future, although with the Fed back in 'helicopter' mode for COVID it's a good bet they will be low at least through 2021 and likely 2022
It's too much work to determine by hand which of these is currently ahead; use the calculator.
If the 'separate paper' is a will, does not need probate and the total bonds in her mother's estate aren't over $100k, https://www.treasurydirect.gov/indiv/research/indepth/ebonds/res_e_bonds_eedeath.htm appears to apply and your wife can be both the Voluntary Representative and Person Entitled. If it's not really a will, but there is no conflicting will and no other potential beneficiary (e.g. a sibling) who might contest this, you might be able to construe it as a will. However, probate law (like much other) is done by each state, not Federally, and varies, so you should consult a lawyer to make sure what you're doing is allowed -- and documented to be so.
If there is a real will, or probate is otherwise needed, I'm pretty sure that trumps any separate, unregistered document about the bonds, and you need to go through whatever legal process applies in your state. But again this is really a question of law not finance.
Sorry for your loss and best of luck.