This is awful, being forced to buy both a sword and a shield and being made to upgrade them at the same time. It was so much better when we had to spend a little more than every other class.
Yeah, I sure loved needing to get two upgrade items for my weapon slots on PLD instead of just one. I can't see why anyone would find that problematic.

Here's what you do:

1) buy the shield (bringing up your total law expenditure to exactly the same as what it would be for every other job)
2) upgrade the sword+shield
3) throw the shield away so you can keep using your precious i175 one + just pretend that you bought some other job's weapon so that the laws/seals you needed was exactly the same as it would have been for every other job

Simples!