Hi - I agree with you that the 1748 baptism is the most likely to be the correct one for underage William.

BL indexes:
Burial: KELLY, Jane, Michael 24 Jan 1752, QUAYLE

Will: KELLY, Jane, 1752-1 Michael A 21 0106220

Summary from Manx Notebook:
1752-1 A 21 KELLY Jane quayle d 23 jan 1752;sis magt (margt quayle in decree - also supv of ch);two unnamed ch ua;husb thos exor;1757: wm quayle sworn supv in place margt quayle (there is an unreadable xd thro paragraph);1772: isabel kelly (one of ch robert acks from bo wm kelly - also wm caine a pledge for fa thos - also discharge sd brother wm caine ?;1754: thos kelly widower of jane als quayle late of baark (half to be used for benefit of wm kelly orphan)lately married ann dawson;john dawson + wm caine,miller, become bound - they later petn that thos kelly has not maintained property + squandered goods;inv;large amount of petns etc

Manx Notebook CD:
Marriages, Michael:
16 Jul 1754 KELLY Thomas, DAWSON Ann, tk booark

Bapt (one of several for this couple's children):
17 Aug 1755 John [s/o] Thos KELLY, Ann DAWSON, tk baark;m Mic 17540716

Sue