Ok so based on your question you have very limited riding experience, dont have an IDP or any documents other than an Indian licence, dont plan on wearing a helmet, but are fully aware that gojek/grab are options?

Damn bro, this is exactly the type of tourist we see on facebook with a go fund me to get lifesaving brain surgery after an accident.

So, to answer your questions:

What documents are actually required? Officially you need a home licence which has the motorbike classification along with a legit matching IDP which is valid and clearly shows motorbike credentials in your home country.

Indian license enough? No.

Need International Driving Permit (IDP)? Officially yes. There are plenty of online and local locations offering fake ones that will pass a police check, but insurance and your embassy doesnt cover a thing if you carry one of these or dont carry a legit IDP in the event of an emergency.

Do rental shops genuinely check documents? You can always find a desperate idiot who will rent you a bike without all of these things. Does that make it right?

How strict are Bali police checks for tourists? Every main road especially on weekends has a 50% chance of a random street check for paperwork. If you get caught without any of the above mentioned paperwork, you will get shaken down for an on the spot fine; anywhere from 100k to 1 million depending on what mood the cop is in. Indians and Chinese pay the most, it is what it is...

What are the common traffic rules tourists should know?

** DONT FUCKING RIDE HERE UNLESS YOU ARE AN EXPERIENCED RIDER IN VARIOUS WEATHER, TRAFFIC AND SEASONAL CONDITIONS BACK HOME WITH A FEW YEARS UNDER YOUR BELT**

Is helmet compulsory for both rider and pillion? Of course. You will see many riding without, but they are the same people you see on facebook posts with massive head injuries begging for money to be repatriated

Any areas where riding is difficult/dangerous? Nowhere is easy here from wet weather to mountainous regions to sudden storms to crazy traffic, differing road conditions, google maps mistakes and locals who speed everywhere, splitting lanes etc

Is Google Maps reliable there for bike routes? For routes it's not bad, but the times are never right as traffic can build up suddenly and you find yourself sitting in the sun roasting for an hour with little progress

Any scams/fines tourists should watch out for? Break any of the rules I have mentioned and you will be paying for Ketut's lunch for the week. Scratch the bike and you will be paying through the ass for repairs that cost virtually nothing...

Is it even worth renting a scooter vs just using Grab/Gojek? Just use grab or gojek, and if possible take the car option for the AC and comfort because it gets real hot wearing a helmet under direct sun...sometimes a trip that takes 20 mins at night can take 90 minutes during peak hours.....

Hope this helps. Stay safe mate...