Funnily enough I also just so happened to look into the Great Marsh very recently and your old post helped a lot with the old save editor and its info.
Basically the only thing you're missing with your observations on what bytes get changed is that a pokemon appearing in a specific area does not change the whole byte, but only 5 bits of it.
Read as little endian 80 ae 36 b7 becomes b7 36 ae 80 or in binary 1011 0111 0011 0110 1010 1110 1000 0000.
If we now split that into units of 5 bits we get 10 11011 10011 01101 01011 10100 00000 or 2 1B 13 0D 0B 14 00. The 2 shouldn't be used for Great Marsh calculation here.
These values line up with the pokemon you had in your areas if we assign the 5 bit blocks "in reverse". 1B is area 6, 13 is area 5, 0D is area 4, 0B is area 3, 14 is area 2, 00 is area 1.
The following table shows which Pokemon each 5 bit block corresponds to in both DP and Pt once you have the national dex.
Hex
Bin
Dec
DP
Pt
00
00000
0
Croagunk
Toxicroak
01
00001
1
Skorupi
Kecleon
02
00010
2
Carnivine
Kecleon
03
00011
3
Croagunk
Carnivine
04
00100
4
Skorupi
Skorupi
05
00101
5
Carnivine
Croagunk
06
00110
6
Golduck
Quagsire
07
00111
7
Croagunk
Drapion
08
01000
8
Skorupi
Skorupi
09
01001
9
Carnivine
Croagunk
0A
01010
10
Roselia
Quagsire
0B
01011
11
Staravia
Kangaskhan
0C
01100
12
Toxicroak
Paras
0D
01101
13
Drapion
Drapion
0E
01110
14
Exeggcute
Exeggcute
0F
01111
15
Golduck
Exeggcute
10
10000
16
Staravia
Skorupi
11
10001
17
Croagunk
Croagunk
12
10010
18
Skorupi
Skorupi
13
10011
19
Carnivine
Carnivine
14
10100
20
Yanma
Yanma
15
10101
21
Shroomish
Shroomish
16
10110
22
Paras
Paras
17
10111
23
Kangaskhan
Kangaskhan
18
11000
24
Gulpin
Gulpin
19
11001
25
Roselia
Tropius
1A
11010
26
Staravia
Gulpin
1B
11011
27
Croagunk
Shroomish
1C
11100
28
Skorupi
Skorupi
1D
11101
29
Carnivine
Carnivine
1E
11110
30
Roselia
Croagunk
1F
11111
31
Golduck
Tangela
I know that in Platinum, every Pokemon that isn't in the Sinnoh Dex gets replaced with Wooper before you get the National Dex. I don't know how it works with DP, as I was playing Platinum while looking into this.
I also went and tried this with manual hex editing of my save file (and fixing the checksum with PKHeX) and changing the area encounters worked like a charm.