Besides what Spaz said, the "129" may potentially be a public demonstration simulator term; rather than putting potentially classified data in public view (how far away an F-35 sees a specific MiG-29 variant) they might just have a generic MiG-29 aircraft with (eg) an RCS of exactly 10m^2 or whatever, with the Lockheed programmers just taking a bit of a shortcut and naming the database entry "129". Alternatively, "129" may be the brevity form of that specific MiG-29 variant, a bit like how in the RWR display of F-16s or F/A-18s, etc something like an SA-10 will just show "10". Here there might be something like "129" for the MiG-29A, "229" for the MiG-29S, "329" for the MiG-29SMT, etc.
Yep. The confidence should be clearly higher, thanks to better sensor and fusion than in a 4 gen.
Something of interest to note too is that (according to the 2018 AIAA paper "F-35 Information Fusion") the F-35's combat ID sensor fusion system works pretty much entirely in probabilities, whereas legacy systems will have some level of probability involved when they're filtering signals or imagery, but then just make an outright declaration to the pilot via some heuristic method.
This means that an F-35 can give tentative / partial answers to the pilot, whereas a legacy jet would continue to show an unknown contact, and it means that an F-35 pilot will be able to identify that an ID is only somewhat certain, whereas a legacy jet's pilot may be mislead if the combat ID is tentative and possibly incorrect (due to advanced enemy EW or whatever). The F-35 combat ID system also utilises a taxonomy system, so it can identify a target to varying levels of detail, whether it's a vague "air" target, or an "enemy air" target, or an "enemy fighter", or an "enemy Su-27", or an "enemy Su-27UBK".
They even go as advanced (in the background; invisible to the pilot) as to have each individual sensor report the uncertainties / error margins for the different dimensions of target track, etc; so the system can do things like identify that it specifically has high uncertainty of (eg) target distance, and then in-turn utilise a better ranging sensor (the radar vs the EOTS for example), or tell a wingman's sensor fusion computer that it lacks that data (and would appreciate the wingman obtaining that data if it has sensor duty-cycle to share).