Is it safe to bottle with champagne yeast?

I have an IIPA going with San Diego Super Yeast (WLP090). It is all grain and I mashed pretty low (148-150) for 90 minutes thinking I would get pretty good conversion. My efficiency was much higher than expected and my OG was around 1.095 or maybe a little higher even. After four weeks in primary it only got down to around 1.018 and I was hoping for it to get a little drier.

Right now I have it wrapped in a warmer to get the temp up a bit, hoping that will help the yeast finish the job (I racked it to secondary, with dry hops, but I washed and re-pitched some of the slurry from the primary).

When dry hopping is done, I was planning to cold crash with gelatin and then throw in some champagne yeast that I have sitting around that I never used (WLP715) along with the bottling sugar just to make sure I get adequate carbonation - I have been having some issues with hit-and-miss carbonation and I am inclined to blame the fact that I have been overshooting my OGs and my yeast just can't do anymore by the time it gets into the bottle.

My real question (other than soliciting general advice for how to get my beer to dry out a bit more) is - if I pitch champagne yeast to a slightly under-attenuated beer, is that a recipe for bottle bombs? I am going to primarily use swing-tops which can hold quite a bit of pressure, but I was also going to bottle some regular bottles in case I want to enter a competition. Is there anything that SDSY won't eat that WLP715 might eat? They list the alcohol tolerance for WLP090 as "high", but they don't really specify a percentage. My beer is currently between 10% and 12% ABV depending on what the OG actually was; does that seem too high for WLP090?

Another possibility - if I pitch the champagne yeast first, before I do the gelatin cold crash, would there be any left for carbonating? Or does the gelatin pretty much remove it all? That's why I was thinking of adding it after the fining process (because I am paranoid all the yeast will be dead or gone), is that a misguided belief?

You have 81% apparent attenuation (or maybe a bit higher as you suspect).
That's definitely within the spec of the yeast (76-83%)

FG of 1.018 is also perfectly within the IIPA style, so you are covered there.
FG of 1.018 doesn't necessarily mean that your beer is "sweet". Did you taste it?
The unfermentable sugars (assuming fermentation is 100% complete) are long chain saccharides and have little flavor/sweetness but they do contribute to perception of body and mouthfeel.

I say taste your beer and see if it's actually necessary to attenuate it more.
Personally I'd leave it alone and package it as is.
But that's your call.
Assuming fermentation is complete champagne yeast will do nothing as there are no simple sugars left to "munch" on unless you add simple sugar for carbonation when packaging.

My hunch is your beer is done fermenting.

I did taste it. It tasted great, but it was a little bit sweet. I mashed low because I was going for a really dry, light-bodied beer with IIPA hoppiness. Maybe I will add a bit of hop tea at bottling to balance it out. Thanks for your input.

Champagne yeast does not ferment more than an ale yeast...in fact, it might ferment less. If there are fermentable sugars still in the beer it will ferment those, but it sounds like you are not really under-attenuated, so i think you will be fine. Champagne yeast is more tolerant to adverse conditions like high ABV and low oxygen/nutrient levels, which make it great to kickstart a stalled fermentation and for bottling big beers. But it won't do anything if there are no more fermentable sugars in there.