Anisotropic Defixation

Berry use triggers your custom status with the defined duration, the status ticks at intervals you defined. If you want an example, check SC_L_LIFEPOTION (Medium Life Potion, ID 12459) or any other similar status.

For stuff like that you're better off creating a custom SC_ status.

There's already bonus2 bHPRegenRate/bSPRegenRate which do exactly that. Check the documentation.

Remove that preserve change from the main block as well as the skill itself, and add it below that block, all by itself.

case ST_PRESERVE:
    if (sd && sd->sc.data[type])
        clif->skill_nodamage(src, bl, skill_id, skill_lv, status_change_end(bl, type, INVALID_TIMER));
    else
        clif->skill_nodamage(src, bl, skill_id, skill_lv, sc_start(src, bl, type, 100, skill_lv, skill->get_time(skill_id, skill_lv)));
    break;

Limit the skill's cast range in skill_db.conf and skillinfolist.lub (clientside), as it's 18 cells by default. As for your other questions, there isn't any noticeable impact as far as I know.