just to add a bit to the AHD etymology, witness OED2
The colonel was so called, because leading the little column or company at the head of the regiment’ (Skeat). The early Fr. coronel (whence also Sp. coronel) was due to the dissimilation of l–l, common in Romanic, though popular etymology associated it with corona, couronne crown. It is still dialectal (see Littré), but was supplanted in literary use, late in 16th c., by the more etymological colonnel; and under this influence and that of translations of Italian military treatises colonel also appeared in Eng. c 1580.

it seems that what we've got here is orthography which reverts to the original etymology and orthoepy which may trace to the French affectations of the Court.

as to lieutenant, OED becomes dog-gone whimsical:
The origin of the btype of forms (which survives in the usual British pronunciation, though the spelling represents the atype) is difficult to explain. The hypothesis of a mere misinterpretation of the graphic form (u read as v), at first sight plausible, does not accord with the facts. In view of the rare OF. form luef for lieu (with which cf. esp. the 15th c. Sc. forms luf-, lufftenand above) it seems likely that the labial glide at the end of OF. lieu as the first element of a compound was sometimes apprehended by Englishmen as a v or f. Possibly some of the forms may be due to association with leave n.1 or lief a.
In 1793 Walker gives the actual pronunciations as (lEv-, lIv"tEn@nt), but expresses the hope that ‘the regular sound, lewtenant’ will in time become current. In England this pronunciation (lju;"tEn@nt) is almost unknown. A newspaper quot. of 1893 in Funk's Standard Dictionary says that (lEf"tEn@nt) is in the U.S. ‘almost confined to the retired list of the navy’.]

(does the Brit. army have the rank of lieutenant colonel?!)