How can I use artificial flavors when making homemade hard seltzer?

Methanol will not be an issue, just try it, if it is horrible then bin it. Costs nothing...

Personally I would use VG (Vegetable Glycerine) based flavours as some people can have reactions to PG (Propylene Glycol) when vaped, but don't know about them when consumed plus the fact it will be so dilute may not even matter.

I might give this a go with forced carbonation as I already have the kit and flavours, Thanks.

methanol is ALWAYS produced by fermentation.

At ABV's under 20 percent or so it is not an issue, only when you are distilling does methanol become an issue, as the distillation process concentrates the alcohol (by removing the water from the solution) so you will be increasing the methanol content for every mouthful. that is why they talk about heads, hearts and tails in distilling.

A glass of wine contains about 0.0041 to 0.02 percent of methanol by volume. The same glass will have about 10-15 percent ethanol. Methanol is much sweeter than ethanol, and even a small amount adds flavor. This sweetness is what makes methanol attractive to use in an artificial sweetener.

Most people regularly consume up to 10 mg of methanol per day as part of their normal diet.

Yes, typically non-fermentable flavorings are best added right before bottling (or kegging). I think the TFA flavors count under this- but I'm not really sure. I would certainly double check they're all food safe and sugarless.

The flavorings should typically not have much sugar in them- and if they did I really doubt methanol fermentation would occur here, or occur in any amount that could cause you adverse effect.

The Northern Brewer seltzer kits I have seen use crystalized fruits for flavoring (lime/grapefruit/lemon/etc) in small amounts, like 2oz or so. I have had no luck in purchasing these crystallized fruits in bulk- but there are some amazon listings for crystallized citrus fruit in individual packets.

A little side note on fermenting seltzer:

It was harder than I thought to get a 1.035 seltzer to run down to 1.000. I've never seen EC-1118 (champagne yeast) get stopped by anything before- but it will stall without good oxidation & nutrient. Have an extra pack of yeast on hand and make sure you add nutrient at least twice during the first week or so of fermentation.