Headlights on High beam will help absorb any spike on disconnection but no guarantee.

Wait a while before disconnecting cars as the flat cars alternator will be working overtime to charge the flat battery and run the car at the same time. Instant disconnection of the leads after starting can cause the spike your trying to avoid.

Yes, connect +ve on batteries and -ve to Block - the resistance assists reducing any spike also.

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Arguably my Automobile is Awash with Alliteration - Subaru Sti S202