Another way to look at it, if they won't let you pay the difference yourself, is to accept that the company for which you are working is not prepared to pay more just so you can earn miles - afterall, the miles are worthless to it and do nothing to increase or improve your efficiency. Afterall, you are already in C, so you have most of the benefits being elite brings you.

You are travelling to 13 countries - time to diversify your alliances, and maintain a programme in each which is favourable to you to cope with the vagaries of 'least expensive' flights.