Craft Request Filter Query Matching Drop Down Fields and Checkbox Fields
I'm probably late to answer this, but I've recentry encountered same problem and here's the solution I came to:

use params for dropdown fields and search for checkbox fields

So my code looks similar to this:

{% set orderParams = {} %}

{% for key, val in craft.request.query %}
    {% if val is not empty %}
        {% if key=='orderFinance' %}
            {% set orderParams = orderParams|merge({orderFinance:val}) %}
        {% elseif key=='orderStatus' %}
            {% set orderParams = orderParams|merge({orderStatus:val}) %}
        {% endif %}
    {% endif %}
{% endfor %}

{% set flagSearchQuery = [] %}
{% for flag in craft.request.query.orderFlag %}
    {% if flag is not empty %}
        {% set flagSearchQuery = flagSearchQuery|merge({query: 'orderFlag:'~flag}) %}
    {% endif %}
{% endfor %}

{% set orders = craft.commerce.orders(orderParams).search(flagSearchQuery).limit(100) %}