For the record, he asked me whether "there is a way to fix the knock back gravity and jump descent". I could not see what there is that needs "fixing" in them, so I answered accordingly.
The question posted in this thread is different.
Anyway, what you are looking for, re: damage-knockback, is this code:
$D36F AD 20 04: lda ObjectFacingLeft
$D372 F0 06: beq + ; $D37A
$D374 A0 FF: ldy #$FF ; Set X velocity to $FF00, which translates into -1 pixels per frame.
$D376 A9 00: lda #$00
$D378 F0 04: beq ++ ; $D37E
+ $D37A A0 01: ldy #$01 ; Set X velocity to $0100, which translates into +1 pixels per frame.
$D37C A9 00: lda #$00
++ $D37E 20 6A D3: jsr $D36A
$D381 A9 FD: lda #$FD
$D383 A0 80: ldy #$80
$D385 20 62 D3: jsr $D362 ; This sets actor's Y velocity to $FD80, which translates into -2.5 pixels per frame.
$D388 A9 1F: lda #$1F
$D38A 20 87 DD: jsr SetObjectUnknown40E_to_A_for_Simon
$D38D 20 CA DC: jsr DeleteSimonWhipObject
$D390 A9 06: lda #$06
$D392 8D D8 03: sta ObjectCurrentActionType
$D395 A9 20: lda #$20
$D397 8D B0 04: sta ObjectUnknown4B0
$D39A 20 D3 D3: jsr FindIfThereIsAFloatingPlatform
$D39D 90 07: bcc + ; $D3A6
$D39F A9 00: lda #$00
$D3A1 9D 7A 04: sta Ending_PrimaryActionIndex,x
$D3A4 85 41: sta $41
+ $D3A6 A9 5F: lda #$5F ; The UGH sound effect (DPCM)
$D3A8 20 18 C1: jsr AnyBankPlayTracks
$D3AB 60: rts
For finding what Simon and other actors are walking on, the game maintains a table of cached level map / solidity in RAM addresses $520 through $6FF. I am not sure how exactly to index that map, but there you have it. It appears to have a horizontal mirroring scheme to it: it stores two screens vertically, and the same screen horizontally twice.
Each tile is indicated as 4 bits (one byte encodes two 16x16 tiles), and a value 0x0 means air, 0xA means solid, 0x3 and 0xC may mean a door. 0x2 may mean a slanted surface (\, ceiling). 0x5 is an "evil" wall (breakable wall). It is also possible that the values are 2-bit, in fact. Functions $1EB37 and $1EA17 in conjunction seem to calculate an offset to this array, invoked by function $1E979.