I think you mostly have it right, but for further exemplification, here are the most common immediately following collocates for each these words from the COCA:
Most common collocates for mandatory
1 TESTING 151
2 MINIMUM 125
3 RETIREMENT 107
4 MINIMUMS 100
5 REPORTING 87
6 SENTENCES 78
7 EVACUATION 74
8 SENTENCING 74
9 DISCLOSURE 63
10 ARBITRATION 56
11 SPENDING 41
12 DRUG 36
13 SENTENCE 35
14 EVACUATIONS 32
15 ARREST 27
16 PRISON 27
17 COUNSELING 26
18 OVERTIME 24
19 RECYCLING 23
20 BUSING 22
21 SANCTIONS 22
22 INSURANCE 21
23 SERVICE 21
24 HIV 20
25 RELEASE 20
Most common collocates for compulsory
1 EDUCATION 86
2 MILITARY 53
3 SCHOOL 31
4 SCHOOLING 21
5 STERILIZATION 19
6 LICENSING 18
7 ATTENDANCE 18
8 NATIONAL 16
9 SERVICE 15
10 MILITIA 14
11 MEMBERSHIP 14
12 JURISDICTION 11
13 HEALTH 11
14 HETEROSEXUALITY 10
15 LABOR 10
16 ARBITRATION 9
17 LICENSE 9
18 FIGURES 9
19 PROCESS 9
20 PUBLIC 9
21 RETIREMENT 8
22 DISPUTE 7
23 ARMY 7
24 VOTING 6
25 INSURANCE 6
Most common collocates for obligatory
1 SERVICE 10
2 NATURE 8
3 STOP 8
4 REFERENCE 6
5 STRUCTURE 5
6 CELIBACY 4
7 EXCHANGE 4
8 VISIT 4
9 READING 4
10 MILITARY 4
11 QUESTION 4
12 SLAP 3
13 NOD 3
14 RETURNS 3
15 BASIC 3
16 PERIOD 3
17 UPON 3
18 PHOTO 3
19 BOOK 3
20 EARLY 3
21 PARTHENOGENESIS 2
22 PREPOSITION 2
23 PILGRIMAGES 2
24 HAGGLING 2
25 ALTRUISTIC 2
As you can see, mandatory and compulsory have much stronger collocates. The collocates for mandatory are more things that are simply required by law, policy, or rule, whereas compulsory is more for specifically things that someone must do (e.g. education, military, sterilization). Obligatory, on the other hand, doesn’t have much of a strong affinity for certain things, and can be used generally for anything that is required by social custom.
Settlement date. None of the regular dictionaries seem to have it, but ones focusing on English business jargon do.
The Free Dictionary
The date on which payment is made to settle a trade.
Investopedia
...the date by which a buyer must pay for the securities delivered by the seller
The latter example highlights one issue with the term, though: it's primarily used for when a payment is made to purchase a security (i.e., a stock or bond).
Best Answer
I think binding (which, according to NOAD, means impose a legal or contractual obligation on) is the more fitting word here. Mandatory simply means something is mandated (such as a mandatory helmet law for motorcyclists); binding implies that something (such as a date) has been agreed to by both parties.
As for alternative suggestions, you might consider contractual deadline, but I'm no expert in contract law. A lawyer could probably give you a better answer.