1. This is pretty common. Those that fall into this category will typically attend the memorial which is sorta the JW version of passover/easter/communion that happens annually and usually coincides with the full moon prior to easter. Magnum summed this up pretty well.
2. This could depend on a number of things. For one, if the husband is seeking reinstatement and the wife is not, that could potentially hold him back somewhat because of the misogynist views of the cult - he should have his wife "in subjection" to him. I can't speak for the experience of women in the cult, but that may have it's own unique challenges too if she were trying to return without her husband. Since being DFed for giving your kid blood isn't something that's likely to be an ongoing problem (i.e. if you were DFed for wife-swapping, one spouse's failure to return might cast suspicion that the DFing offense is continuing) so it probably wouldn't be insurmountable if only one person decided to return. This can also depend on the 3 elders that are handling the case - there are a wide range of personalities and some are more harsh than others.
If one were reinstated without the other, I suspect it would put a fair amount of strain on the relationship, even if both still somewhat believe the doctrine. The one that returns would likely place their "spirituality" (i.e. involvement in cult activities) ahead of the needs of the family. Furthermore, they would likely try to drag the kids to cult meetings and in service (door to door recruiting) which would probably be a source of frustration as the kids will begin to be indoctrinated to have a poor view of the non-attending spouse. The children would likely begin telling the non-attending spouse that they're going to die at armageddon or perhaps using their non-attendance as a reason that they have no authority over them. The kids would probably be the largest source of conflict in most cases. Though the spouse that returns to the cult would likely (whether consciously or subconsciously) blame any issues in their relationship or in their lives in general on their spouse's non-attendance, and if the wife is the one that returns (which is more common) she would likely resent her husband for not being the "spiritual head" that she needs and would blame him for any difficulty she has in maintaining her cult activity. A woman in the cult also has very limited options for obtaining any social status among the members without a husband that's also in. Women can really only pioneer, which I suspect would be unlikely in the case of one spouse going back and the other not, so without the status of being an elder's wife, she would be at the bottom of the totem poll.
3. In general 6 months would be really quick, 1-2 years is more typical. Not sure of the specifics around blood - it might depend on the elders handling the case. If they have young children they may be more compassionate and be more inclined to reinstate, but the blood doctrine is one that his held in very high regard so a violation there might draw out some vindictiveness. You could go either way with it and have it be believable. If the kid given the transfusion still died, I think that actually might make their reinstatement go a little more quickly since the couple may well believe their child's death to be a punishment by god and this would result in what is probably going to be a more genuine display of grief and remorse, which is one of the things the elders will look for.
4. The plot was kinda lightly described but I don't see any significant holes.
5. In the time period that you're depicting, I think it's very likely that they'd leave and still hold the beliefs. Without the internet it was very difficult to come by the information needed to destroy the doctrine, and took a great deal of time and mental investment to do so. So most that walked away probably were left with some degree of belief remaining, they were just making the choice to be happy now and not wait for paradise. Or, as magnum said, they just couldn't quit smoking or gambling or whatever else.