Is this a subjunctive form of (ku)leta? If so, is the subject prefix optional (I'd have used 'ulete')? That feels unusual to me.
Kuleta is just an irregular verb. The imperative is not "leta" but lete for some reason, like how kwenda and kuja have the irregular forms nenda and njoo.
Could one say "lete chumvi tafadhali" at a dinnertable (please pass the salt)?
you don't typically use 'lete' when being polite; it's used when being very casual or demanding (and 'lete' would be interpreted as 'bring' rather than 'pass')
'Please pass the salt' would be 'Pitisha chumvi tafadhali'