How does the 180 rule work for UK tourist visas?

I'm not an expert but I wouldn't advise doing any of this. There are lots of stories on this site of people applying for a visa for, say, a three week visit, seeing that their visa is valid for six months and deciding to stay that long. Then they're refused entry at the border because what they said on their visa application was wildly different to what they tried to do.

Your scenario has the same feel to it, except that the planned visit was longer so the extension isn't as extreme. But, basically, you're saying "Oh, wow, it says six months so I can stay that long! Cooooool!!"

You're not allowed to use repeated trips on a visitor visa to effectively live in the UK. You're proposing that your parents will spend approximately nine months out of a nine-and-a-half month period in the UK with you: that looks a lot like living in the UK. You're then proposing that they return to India for a couple of weeks and then come back yet again, which sounds even more like living in the UK.

1. Does their 180 days period gets reset when they left UK for Europe tour?

The six-month period applies to each admission to the UK, unless the immigration officer decides to admit them for a shorter period – or even not to admit them at all.

2. Can I postpone their return travel to India by 6 months (Approx. Mid Sep 2019) after their return to UK on 26th March 2019?

You can but it would be a bad idea. It's a real possibility that they will not be given a six-month entry when they return to the UK after their European trip.

3. Is the 180 days criteria is Max. stay criteria in one year ? OR are they eligible to return to UK after spending 5 Months in UK within one year?

Six months is the maximum length of a single entry. There is no formal restriction on how many visits can be made or how long there has to be between them, but you are not allowed to live in the UK by making repeated visits on a visitor visa and, if they suspect you're doing that, they won't allow you to enter the UK.

4. Lastly, can I re-book their return travel to UK from India in End Sep 2019?

You can do but that would be an extremely bad idea. By that time, they will have spent more than eight out of the last nine months in the UK and coming straight back again will look like they're living in the UK and just leaving every now and again to reset the counter.

However, during this period they left UK on 16th March 2019 for Europe tour and will be returning back to UK on 26th March 2019.

If they are readmitted without any special annotation then they get another 6 months. The previous stay does not count anymore. However, an intention to stay in the country for 11 out of 12 months may well be a big red flag for a secondary interview.

It does sound like trying to live in the UK for extended periods of time through multiple successive visits - something they clearly state a visitor shouldn't do.

Awaiting your confirmation, in order to plan their retun travel to India and then coming over again to UK.

After planning to spend 11 out of 12 months in the UK, you intend to have them travel again to the UK soon after, once again. This may very well get them into trouble at UK border. I certainly think it is not advisable in ordinary circumstances.

_{I'm sorry if i read the 11 out of 12 month thing wrong, but that doesn't matter. Any extended stay with frequent successive extended visits still matters the same.}