I think people are over thinking this. It's likely any required sensors are built into the tablet and that to compensate for drift it still makes use of a external sensor bar but drift may also be compensated for simply by a reset position switch. Nintendo know how to do cheap motion tracking.
The original Oculus Rift developer version had a 8" screen and only 720p and still provided a good VR experience.
We don't know the resolution of the Switch for sure it could be 720p or 1080p or a resolution in between. I would prefer 1080p but 720p is not a deal breaker and there are techniques for removing the screen door effect. A soft focus can be applied to the optics section to prevent screen door and the optics can present more detail to the centre of the image rather than the outer area.
Absolutely no reason the Nintendo VR system can't be both cheap with good performance. The main limitation is the processing power of the tablet itself but it should still be enough to provide last gen games in VR which to be perfectly honest I'm happy with. Skyrim could well be one of the first VR games. I'd rather play Skyrim in VR than 4k.
Lots of people are using their 720p or lower resolution phones with google cardboard.
Looking at phones with 1080p displays on aliexpress there is a difference of £15 approx between the minimum price of a 1080p phone compared to 720p phone. Something like £40 for 720p basic quad core phone and £55 for 1080p and with these retail prices that means perhaps a $7-10 difference in factory cost prices.
https://www.aliexpress.com/item/Free-Shipping-TEENO-5-5-Smartphone-HD-Smartphone-Quad-Core-Cell-Phone-Mobile-Phone/32723941616.html?spm=2114.01010108.3.1.41eI6r&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10084_10083_10080_10082_10081_10060_10061_10062_10056_10055_10054_10059_10099_10078_10079_427_10073_10103_10102_10096_10052_10050_10051,searchweb201603_9&btsid=df6c7b9d-40d4-4d32-ac1c-baf08842e879
