I really like your mnemonic!
Quite a few Modern Welsh words starting with h have equivalents in English and Latin starting with s:
haf ~ Eng. summer
halen ~ Eng. salt, Lat. sāl
haul ~ Eng. sun, Lat. sōl
hafal "equal" ~ Eng. same, Lat. similis
hen "old" ~ Lat. senior, senex
hwyr "late" ~ Lat. sērus (think Eng. serotine)
Hafren ~ Eng. Severn, Lat. Sabrina
Thanks. So I'll report the fact that "in summer" wasn't accepted for "yn yr haf"then.