Based on my experience, it's best to let your battery charge until it's full.
It completely depends on your usage behavior and preferences. If you are on the road a lot and want to get the most out of the battery, then it makes sense to charge it to 100% every time. However, it is a fact that the battery wears out way faster when charging to 100%. You can avoid this extreme load and wearout by using the Deck mostly between 20% and 80%. So do not discharge below 20% and do not charge above 80%.
As far as I know, the Deck notices when it is connected to the power supply for a long time and discharges itself to 80% to save the battery. If the Deck remains connected to the power supply and you play, the battery is bypassed and the Deck is powered only by the power supply. Ideal for a continuous use as a PC on the dock.
Letting your battery charge until full before using it will also extend its life.
No. The battery wears out faster when charging to 100% every time.
Another tip: Don't use your device immediately after it's fully charged. You should wait for at least an hour so your device don't overheat.
Where did you get this information from? That doesn't make much sense.
The worst case scenario in terms of temperatures would be if the Deck is using the fast charge function (up to ~80%, after that, the Deck charges slowly to reduce battery wear out) AND you are playing a demanding game, so the power supply is pumping the full 45w into the Deck. And even then, the charging-IC is sufficiently protected. It only gets a bit over 90°C, but according to the spec sheet, it is allowed to get much hotter (up to 130°C IIRC). Before anything starts to burn, the charging-IC shuts down and the power is cut.
Additionally, since we're on the topic, you should still watch your device from time to time while it's charging so it won't overheat and maybe cause a fire.
Yes ok, if you are a bit paranoid and really worried about having a defective Deck, you should keep an eye on it when charging. Just like you would on your cell phone, laptop, bluetooth speakers, game console controllers, tablet, electric toothbrush, razor, Switch, and any other device with a built-in battery. A healthy device will not burst into flames.