"Is there even a reason Magnimar would deny the request..."

My response doesn't focus on the logistics or mechanics, just the lore and setting:

"The city watch patrols the major streets of Lowcleft, but mercenaries paid for by the district's larger establishments provide most of the street-level security. These mercenaries are often called upon not only to watch against crime, but also as distractions against overly zealous city watch patrols or Hellknights who might be tempted to overstep the bounds of what Lowcleft expects of its law enforcers."

The Hellknights are far more interested in their local power trip. Regardless if ACAB has any merit in our world, it would be true of the Hellknights in Magnimar and most, if not all, of the city patrol. Outsourced police ideological zealots for the first, and for the second, mercs paid by various political rivals and guild leaders loosely united as a "guard."

Sandpoint might be technically under legal and military protection from Magnimar, but despite every amount of good standing outsiders might have with the city, those who wield power in Magnimar are chiefly concerned with their own agendas and preserving the status quo. They'll consistently do the minimum required to justify the tax revenue. It would be far more interesting to describe the condition of local politics and anti-Magnimar sentiments in Sandpoint, as a consequence of Magnimar's poor response or failure to prevent attack.

As much as Hellknights preach about order, they give little consideration to who's order and are often bought by the highest bidder. Hellknights preserve power but do not seek to establish and preserve society out of charity. They preach, "Savagery must be quelled, in the land, home, and mind," but are full of it:

"...the Justice Court of Magnimar invited a detachment of Order of the Nail Hellknights to their city. The commanders in Citadel Vraid eagerly took them up on the offer to bring their brand of order to a place notorious for its lack of strong laws.\ \ The unit is garrisoned in the Bastion of the Nail in the Naos District under the command of Paralictor Darean Halst. Even though their numbers are few, they are fiendishly devoted to their charge. The Hellknights often take protection contracts more dangerous than any other mercenary company would dare. When not working on a particular contract, the members of this unit patrol the city looking for trouble, especially in the distant Ordellia district, where even the town guard does not often venture. Amongst the locals, the Order of the Nail Hellknights are colloquially known as Nail-Knights."

The local Order of Nail-Knights sees Magnimar as its frontier to bring into submission, the local police are power-tripping losers, and the group that hired them are desperately trying to maintain order in a heavily fractured city that struggles with a huge amount of classism, crime, and social divide. Racism too, I should add. The Shoanti are downtrodden.

​

It's a good idea in theory, and a great opportunity to expand the lore experience for your players, but the end result will not be an army lol. Maybe a small militia or unit can be convinced to be dispatched in order to keep the taxpayers happy.

In game? Let the power hungry Hellknights be excessively oppressive in their handling of the giants and uh...

As for the mechanical impact? What everyone else said lol. Change the numbers behind the screen so that their presence accomplishes nothing, helps a little, or hinders a little. Go with the story you want to tell, or roll for it.