Should I start a new topic to increase my chances of having more detailed answers ?
Depends what kind of answer you're looking for... do you want us to write the mod for you... or give you advice where to look?
If you want me to write it for you, I have to politely decline.
If you want pointers, I'll repeat where the information is stored:
1. mission race weights are stored in alienMissions.rul
2. mission generation information is stored in missionScripts.rul
3. there are two basic modes of mission generation: mission-table-based and scripted... mission tables are located in regions.rul; mission scripts are located directly in missionScripts.rul
4. mission tables approach is inherently random, you can't affect what comes first/next/last
5. if you want more control, you'll need to use mission scripts; or combination of both
6. many mods have changed mission generation, you can find almost any example you can think of by studying vanilla ruleset and other mods
There are several ways how to achieve postponing some missions, the easiest in your case is probably to use scripted missions for a first few months and after that revert to using mission tables. At least, that's how I would do it.
Second quite easy option would be removing these 2 missions from mission tables completely, and only generate them via scripts.