schwierig

Simple modification for Priest.
I am using the Shield Reflect Statuschange from the Crusader and add it as an additional status on the Priest when the SL is cast. Though I can't make it reflect exactly 30% with that method, since it is using this table:
 
http://ratemyserver.net/index.php?sk_name=Reflect+Shield&page=skill_db&sk_search=Search
 
I could make a switch for the status effect itself, but I think that's a bit overkill considering on level 7 the difference is only 1%.
 
In skill.c search for:
if (skill_id == SL_SUPERNOVICE && dstsd && dstsd->die_counter && !(rnd()%100)) { //Erase death count 1% of the casts dstsd->die_counter = 0; pc_setglobalreg(dstsd,"PC_DIE_COUNTER", 0); clif->specialeffect(bl, 0x152, AREA); //SC_SOULLINK invokes status_calc_pc for us. } and place this after that:
if (skill_id == SL_PRIEST) { clif->skill_nodamage(src,bl,skill_id,skill_lv, sc_start4(bl,SC_REFLECTSHIELD,100,7,skill_id,0,0,skill->get_time(skill_id,skill_lv)));}  
I have no idea how the others work. I don't even know if the HP modification is possible. Though I will take a look into the Knight issue. Everything else I can't help you with at this point.

Why aren't you just reading the thread?
 
ITEM_ID:PRICEGrape:100

The very easy ones:
2.Alchemist - Acid Terror Damage + 100% (I guess you want to double the damage here)
search (battle.c):
case AM_ACIDTERROR: skillratio += 40 * skill_lv; break;  
replace with:
case AM_ACIDTERROR: skillratio += 40 * skill_lv; if (sc && sc->data[SC_SOULLINK] && sc->data[SC_SOULLINK]->val2 == SL_ALCHEMIST) skillratio *= 2; break;  
5.Crusader - Shield-Chain damage by 200% (I guess you want to tripple the damage?)
search (battle.c):
case PA_SHIELDCHAIN: skillratio += 30 * skill_lv; break;  replace with:
case PA_SHIELDCHAIN: skillratio += 30 * skill_lv; if (sc && sc->data[SC_SOULLINK] && sc->data[SC_SOULLINK]->val2 == SL_CRUSADER) skillratio *= 3; break;  
8.Rogue - You can use Single Strip through Full Chemical Protection by consumin Glistening Coat(Working only when the target has FCP or Full Chemical Protection), Increase Double Strafe damage by 150%
- Strip:
http://rathena.org/board/topic/87197-single-strip-bypass/
- DS
Not sure if this works (no time to try)
search (battle.c):
case AC_DOUBLE:case MA_DOUBLE: skillratio += 10 * (skill_lv-1); break;  replace with:
case AC_DOUBLE:case MA_DOUBLE: skillratio += 10 * (skill_lv-1); if (sc && sc->data[SC_SOULLINK] && sc->data[SC_SOULLINK]->val2 == SL_ROGUE) skillratio *= 1.5; break;  
9.Hunter - It gives 15% Chance of auto casting Falcon Assault when attacking.
search (skill.c):
// Automatic trigger of Blitz Beat if (pc_isfalcon(sd) && sd->status.weapon == W_BOW && (temp=pc->checkskill(sd,HT_BLITZBEAT))>0 && rnd()%1000 <= sstatus->luk*3 ) { rate = sd->status.job_level / 10 + 1; skill->castend_damage_id(src,bl,HT_BLITZBEAT,(temp<rate)?temp:rate,tick,SD_LEVEL); } place after that:
if(pc_isfalcon(sd) && sd->status.weapon == W_BOW && sc && sc->data[SC_SOULLINK] && sc->data[SC_SOULLINK]->val2 == SL_HUNTER) { skill->castend_damage_id(src,bl,SN_FALCONASSAULT,temp,tick,0); }  
Looking up the other ones and will edit later I guess.