If you don't crawl underneath and breathe in the exhaust fumes from the engine, you'll be fine. If you do crawl underneath and breathe in the exhaust fumes, you'll be dead from carbon monoxide poisoning long before any harmful effects from the tetraethyl lead combustion byproducts in the fuel.
While you don't want to ingest lots of lead, that won't really ...
Any security precaution will be shared with the pilots. Therefore, a determined pilot will know those and be able to find ways to circumvent them beforehand.
What are some of the possible precautions?
Give both pilots their individual key? Easy - disable the lock before the other pilot leaves the cockpit, maybe by breaking off your own key when it is stuck ...
The EASA class two requirements are (p.99):
(2) For a class 2 medical certificate:
(i) Distant visual acuity, with or without correction, shall be 6/12
(0,5) or better in each eye separately and visual acuity with both
eyes shall be 6/9 (0,7) or better.
(ii) Notwithstanding point (b)(2)(i), applicants with substandard
vision in one eye or monocularity may ...
EASA Part-MED regulations are non black and white. There is some margin up to the discretion of the AME and Optometrist. If you are outside of the limits, you may be asked to undergo further examinations.
Corrections (Spectacles/contact lenses) are allowed and mandatory depending on your non-corrected visual acuity.