I'm starting to have second thoughts about whether I think Callum will be killed off over the 60th. Corrie, unlike EastEnders, doesn't seem to have murderers (although the perpetrator any other crime is seemingly fine) walking around all over the place and they get punished to some extent (even Tracey who was eventually released from jail in a farfetched storyline.) I don't think they would want to make any of the Platts a murderer for that reason, as it would make their departure inevitable. As someone else said, Callum could return in the future and cause no end of problems for them. It was something that occurred to me when it started to look as though David and Kylie would actually give him the £20,000. If Callum signed away his rights, he might turn up in the future and claim that they suggested they pay him to do it. The paperwork might then be considered not legally binding and/or he could claim he signed under duress as he had debts. I'm not sure if that would hold in the real world but it should be good enough for a soap that has retconned to accommodate this storyline.
If Callum does end up under the Platts' garage (nice theory by the way) then I think Tony will kill him. He seems like the kind of character that has a shelf life and who they wouldn't particularly regret disposing of for the sake of a dramatic storyline.
I can't believe we didn't see Max kicking off at the school nurse saying that he didn't want to go with Callum. This Denton seems very convenient all of a sudden, as if they changed/rushed the ending of the storyline. Did Callum buy all those presents with money from a loan shark then?