I think the bigger problem is that:
(1) Tesla gave that interpretation of the data. If they wanted to orchestrate a cover up, they could've given another decoding of the bits that had a more favorable interpretation.
(2) Tesla had to recover the data from the computer. If they wanted to hide data, they could've conveniently curated what to give the NTSB or not. Or say that it was just not recoverable.
With that said, those are pure hypotheticals. Obstructing a NTSB investigation sounds criminal, and I would not imagine Tesla doing that for the sake of covering up one death.
1) Is true, but that requires a bad faith manufacturer. It would also potentially work only once (since the next investigation that happens may uncover that inconsistency). As posted by others, it is a felony to obstruct a NTSB investigation.
Even if there is standardization, they are likely to work towards standardizing the minimum generic variables (but that still leaves room for a ton of proprietary variables). NHTSA rule suggests 15 minimum essential data elements recorded (and up to 30 additional). The standardized are going to be generic stuff like airbag activation, acceleration, brake application, etc. The example I gave of autopilot parameters are unlikely to fall under this (it would be difficult to standardize too).
https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/edrfria.pdf
While the FDR for airplanes are standardized, it seems to still allow proprietary parameters to be stored (which may potentially be useful in an investigation). The NTSB would then still be dependent on the manufacturer being honest about the conversion for the proprietary parameters.
https://www.ntsb.gov/investigations/process/Documents/FDR_Handbook.pdf
There are other ways to cheat, even with standardized recorders. You can have the car computer record fake data when it potentially leads to manufacturer liability (for example something that Tesla has been accused of before: when a car crashes into a wall from a standstill, which so far has been 100% driver error according to logs).
2) This is not true. The data was in an internal SD card, which the NTSB was able to retrieve all the data from. The only missing part is translating that data (which required a proprietary tool from Tesla).
From Page 10-5:
"Approximately 510 MB of data was recovered from the vehicle by removal and duplication of data stored on the GTW internal SD card. This data was composed of 87 files organized in 8 folders and stored on the SD card in a MS Windows readable format. The data included 8 image files representing data from the forward facing camera. A small subset of this data was stored in ASCII format. But the vast majority, including the vehicle log files containing all of the parametric data discussed in this report, was stored in a proprietary binary format that required the use of in-house manufacturer software tools for conversion into engineering units."