What about the other way around?

25 watts of Class A power for the LS3/5A's will be fine! I have a pair of Rogers. They are not meant for playing metal at 100 db! More power will only risk damaging the KEF B110 woofers.

Assuming the reverse is true that wattage output will be halved as resistance doubles, would a pair of 15 ohm Rogers LS3/5As (82.5dB/W/M) be too much of a load for a 50 wpc Forte 1A?
With ~25 wpc, should I be concerned about clipping?

You're correct- the amp is considered a 'voltage source' so it will put out the same voltage into any impedance (within limits of course) so if you double the load impedance the output power is cut in half.

This isn't a bad thing- into higher impedances any amplifier will make less distortion and you will hear this as greater detail and smoother sound overall. 25 watts into a set of Rogers is a decent amount of power. The speaker is meant for nearfield listening and so can't make a lot of sound pressure. You'll find that in a bedroom system this is plenty of power. I run an am with only 5 watts/channel in my bedroom system and my speakers are only slightly more efficient than the Rogers; I never come anywhere close to overdriving the amp (a sub really helps)!

"...I understand that an SS amp in general will double (or some percentage of doubling) in output when going from 8 to 4 ohms..."

Generalizations can be a dangerous thing, it's best to go by the specs for your amp than to make generalizations. I think you'll be fine with the Forte1A.

"...should I be concerned about clipping?..."
You should always be concerned about clipping but play at reasonable volumes and you'll save yourself a lot of problems.