Yes.  You can join for free.

Yes. . They cannot be extended, although there are strategies for retaining your Hilton Honors points.  

Members who do not have "eligible activity"  in any 15 consecutive month period may be removed from the Program (and forfeit any accrued points). At the discretion of Hilton Honors, such Members may be permitted to remain in the Program and removed after five years of inactivity (but will still forfeit the points).

Note that with the covid-19 crisis affecting travel so significantly, Hilton Honors has extended the expiry period, so that all Points set to expire in 2021 will not expire until December 31, 2021.

Yes, points expire at the end of the calendar month that is 15 months from when they were earned. However, they can be "refreshed" using strategies for retaining your Hilton Honors points.

Note that with the covid-19 crisis affecting travel so significantly, Hilton Honors has extended the expiry period, so that all Points set to expire in 2021 will not expire until December 31, 2022.

We value Hilton Honors points at approx USD 5 per thousand (0.5 cents each), so 50,000 points would have an approximate value of USD 250.

Diamond status is earned after (within a calendar year) either:

• 60 nights
• 30 stays
• earning 120,000 Honors points

Note that due to the impact covid-19 has had on travel, each of these qualification criteria have been halved for 2021 and reduced by 30% for 2022..

Yes, but only for elite tier members.  The amount of nights above the threshold you used to obtain/retain your status this year is rolled over to help you qualify for elite tier status in the following calendar year. At the end of that following year, they expire.

No.  The amount of nights, but not stays  above the threshold you used to obtain/retain your status this year is carried over.

Free nights can be redeemed for as few as 5,000 points.  Most standard rooms are available for 95,000 points or less, but there are some specialty properties with standard rooms for 150,000 points !  Note that these amounts are minimums, as actual requirements may be more during peak seasons.

Standard rooms at Conrad, Waldorf Astoria and the latest brand LXR are at the upper end of this range.  Tapestry Collection standard  rooms seem to max out at 80,000 points.  Curio Collection properties have plenty of availability at 80,000 points.  Note that premium rooms may require many more points.

The minimum you will need varies considerably among these properties.  Some rooms with a maximum requirement of 80,000 points also have a minimum requirement of 49,000.  Some properties have a minimum that is half the maximum requirement.