2) There are 2 factors here: is it possible? Is it cost effective?
It is possible if there is inventory in Club, it can be just one seat. The historic basis may not help that much if the inventory is limited but it's just the fare difference based on what it was at the time of purchase.

Cost effective - if I for India inventory is available then it's likely (but not guaranteed) to be cost effective. It doesn't mean that particular fare basis is still available, it probably won't be anyway, but the fare difference could be quite small or nil if that fare bucket is still available. If you go up to fare bucket D then there's almost certain to be a larger fare difference but it may still be manageable.

J on the add-on to MXP is quite normal, and there is an add-on (end-on constructions) fare for J which can be reasonably inexpensive provided it's bolted on to a longhaul. Usually that J add on will stay the same, or similar, if you move to another date, it's a set figure. It wouldn't show like this if there was (a) a through fare SEA-MXP and (b) that through fare was cheaper than the fare you booked.

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