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I would suggest a flooded oil pump for this with an overflow reservoir. Your need for a primary and secondary pump to balance the system seems to be a high likelihood of failure. A flooded pump would be a pump picking up its oil from the reservoir and draining back into it. This allows for expansion and loss without adjustments. It also gives you some failure buffer. If you loose a line or have a leak the system could potentially continue oiling for as long as the reservoir has oil. Using a fluid level sensor your can include a low oil shutdown that could save you for catastrophic failure. Sizing the reservoir would largely rely on your thermal displacement and system flow rate. I would estimate a 4 qt reservoir in addition to what the system holds with a 1qt air buffer.
If you use a 10 digit code that uses all the keys then that is totally eliminated. Using the same digit two or more time greatly increases the difficulty using that method also. Bio Metric such as finger print, (Under $100), hand mapping, (More accurate and reliable), or iris (Less affordable) would all be more secure.
You are correct, Most blanks are brass. However many of the cheap door knob and deadbolt sets people buy have aluminum keys included. They are thin and weak and will corrode quickly if left outside. I personally think keys are a very outdated and insecure method. (Not to say they are ineffective.) A password or bio-metric option is much more secure since it involves keys that are very hard to loose or replicate. And it has become far more affordable.
most keys today are aluminum, zink, or steel with a coating. only a few high end brands still supply brass keys.