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Bonjour ! 

J'ai override Group.php et Product.php, j'ai peut-être fait d'autres modifications qui suivent celle-ci, Je ne suis pas encore une pro de prestashop, c'est peut-être pas optimal... Au moins, c'est le coeur d'une solution.
 J'ai simplement rendu possible le passage en paramètre d'un tableau d'id pour récupérer toutes les remises possibles. 

    public static function getReductionByIdGroup($id_group)
    {
        $real_id_group = $id_group;
        if(is_array($id_group)){
            $id_group = implode(',',$id_group);
        }
        if (!isset(self::$cache_reduction['group'][$id_group])) {
            if(!is_array($real_id_group)){
                self::$cache_reduction['group'][$id_group] = Db::getInstance(_PS_USE_SQL_SLAVE_)->getValue('
                SELECT `reduction`
                FROM `' . _DB_PREFIX_ . 'group`
                WHERE `id_group` = ' . (int) $id_group);
            }else{
                self::$cache_reduction['group'][$id_group] = Db::getInstance(_PS_USE_SQL_SLAVE_)->getValue('
                SELECT `reduction`
                FROM `' . _DB_PREFIX_ . 'group`
                WHERE `id_group` IN ('.$id_group.')' );
            }
        }

        return self::$cache_reduction['group'][$id_group];
    }

et dans Product.php j'ai ajouté ces 2 lignes dans la fonction getPriceStatic pour récupérer tous id des groupes auxquels appartient l'utilisateur

        global $cookie;
        $id_group=Customer::getGroupsStatic($cookie->id_customer);

Product.php

 public static function getPriceStatic(
        $id_product,
        $usetax = true,
        $id_product_attribute = null,
        $decimals = 6,
        $divisor = null,
        $only_reduc = false,
        $usereduc = true,
        $quantity = 1,
        $force_associated_tax = false,
        $id_customer = null,
        $id_cart = null,
        $id_address = null,
        &$specific_price_output = null,
        $with_ecotax = true,
        $use_group_reduction = true,
        Context $context = null,
        $use_customer_price = true,
        $id_customization = null
    ) {
        if (!$context) {
            $context = Context::getContext();
        }

        $cur_cart = $context->cart;

        if ($divisor !== null) {
            Tools::displayParameterAsDeprecated('divisor');
        }

        if (!Validate::isBool($usetax) || !Validate::isUnsignedId($id_product)) {
            die(Tools::displayError());
        }

        // Initializations
        $id_group = null;

        // Code prestashop original replaced
        if ($id_customer) {
            $id_group = Customer::getDefaultGroupId((int) $id_customer);
        }
        if (!$id_group) {
            $id_group = (int) Group::getCurrent()->id;
        }

        global $cookie;
        $id_group=Customer::getGroupsStatic($cookie->id_customer);

        // If there is cart in context or if the specified id_cart is different from the context cart id
        if (!is_object($cur_cart) || (Validate::isUnsignedInt($id_cart) && $id_cart && $cur_cart->id != $id_cart)) {
            /*
            * When a user (e.g., guest, customer, Google...) is on PrestaShop, he has already its cart as the global (see /init.php)
            * When a non-user calls directly this method (e.g., payment module...) is on PrestaShop, he does not have already it BUT knows the cart ID
            * When called from the back office, cart ID can be inexistant
            */
            if (!$id_cart && !isset($context->employee)) {
                die(Tools::displayError());
            }
            $cur_cart = new Cart($id_cart);
            // Store cart in context to avoid multiple instantiations in BO
            if (!Validate::isLoadedObject($context->cart)) {
                $context->cart = $cur_cart;
            }
        }

        $cart_quantity = 0;
        if ((int) $id_cart) {
            $cache_id = 'Product::getPriceStatic_' . (int) $id_product . '-' . (int) $id_cart;
            if (!Cache::isStored($cache_id) || ($cart_quantity = Cache::retrieve($cache_id) != (int) $quantity)) {
                $sql = 'SELECT SUM(`quantity`)
                FROM `' . _DB_PREFIX_ . 'cart_product`
                WHERE `id_product` = ' . (int) $id_product . '
                AND `id_cart` = ' . (int) $id_cart;
                $cart_quantity = (int) Db::getInstance(_PS_USE_SQL_SLAVE_)->getValue($sql);
                Cache::store($cache_id, $cart_quantity);
            } else {
                $cart_quantity = Cache::retrieve($cache_id);
            }
        }

        $id_currency = Validate::isLoadedObject($context->currency) ? (int) $context->currency->id : (int) Configuration::get('PS_CURRENCY_DEFAULT');

        if (!$id_address && Validate::isLoadedObject($cur_cart)) {
            $id_address = $cur_cart->{Configuration::get('PS_TAX_ADDRESS_TYPE')};
        }

        // retrieve address informations
        $address = Address::initialize($id_address, true);
        $id_country = (int) $address->id_country;
        $id_state = (int) $address->id_state;
        $zipcode = $address->postcode;

        if (Tax::excludeTaxeOption()) {
            $usetax = false;
        }

        if ($usetax != false
            && !empty($address->vat_number)
            && $address->id_country != Configuration::get('VATNUMBER_COUNTRY')
            && Configuration::get('VATNUMBER_MANAGEMENT')) {
            $usetax = false;
        }

        if (null === $id_customer && Validate::isLoadedObject($context->customer)) {
            $id_customer = $context->customer->id;
        }
        $return = Product::priceCalculation(
            $context->shop->id,
            $id_product,
            $id_product_attribute,
            $id_country,
            $id_state,
            $zipcode,
            $id_currency,
            $id_group,
            $quantity,
            $usetax,
            $decimals,
            $only_reduc,
            $usereduc,
            $with_ecotax,
            $specific_price_output,
            $use_group_reduction,
            $id_customer,
            $use_customer_price,
            $id_cart,
            $cart_quantity,
            $id_customization
        );

        return $return;
    }

J'espère que ça aidera suffisamment