Seller did not perform within the time period as per real estate contract

This is from a juror's common sense perspective, being a homeowner on the selling and buying ends: Read the contract, including the fine print (see the part about you having your own attorney review the contract).
You can bring suit for any reason, valid or not, which is determined in a court of law, by application of the "law" by the judge and/or jury. Losing, you are out lawyer costs - yours and theirs. This is not your typical "on a contingency basis" case, and, even if it were, you would invariably have out of pocket costs other than just your lawywer's fees (Not the type, mainly because the tax credit wouldn't constitute an award worth even an ambulance chaser's percentage). And, don't expect any putative award unless the realtor(s) were just totaly negligent and didn't perform, in which you may have a cause of action against them, and their boss/broker, but, it's still just about the amount of the lost tax credit.
In any case, you can be countersued before you even turn around, for which you would then have to get a lawyer to respond, and, absent any response on your part to their countersuit, the courts could rule in their favor by virtue of default.
Though you say the tax credit is irrelevant ( = doesn't amount to much), since you want to "take action" (defined as sue, or arbitrate, depending upon the terms of the contract), if you do get the tax credit, the courts will balk at any "delayed performance" claim, since you have no loss, just anxiety which is part and parcel of buying a home/real estate. Besides, there is no "delayed", only "non" performance. If not done in five days, "non" applies, and the contract already expired (absent any other provisions). If both agree to extend the time, "non" would still be the correct term for the amended contract.
Now that you have read the contract, as above (I haven't), a typical realty contract will, by any and all means possible, disallow lawsuits because it is written by the seller's agency to protect the agency, well, yes, the seller too. And, unless you have hired a "buyer's broker" to represent you and is paid by you, the agent/agency that you may think works for you really works for the seller. After the five days sited in your addendum, the contract is null and void because they did not comply with its terms, though it could continue if agreeable to both parties. If you even hint at any intent to "take action", don't expect them and their agent, and the one you think works for you, to want to deal any further with you.
You added the short performance period because you wanted to make sure the things to be remedied were done in enough time to qualify for the tax credit. I'm sure they only committed to a dollar cap for at least some part of the remedies, which, if exceeded, is another "no fault" means of voiding the contract. Your home inspector's written report, must have given you a cost and time period on which you based the five days, sufficient to account for the Saturday and Sunday, which means there wasn't that much to do.
Actually, 24 Oct to 2 Nov is nine days, not five. If, as you say, the seller and his agent are doing the work ("repairs/replacements and improvements"), rather than hiring a contractor, you should've been leery of the quality and time.
Supposing, as above, that the work was not extensive, if a contractor(s) had come out and looked at the project, signed a contract, bought materials and equipment, scheduled his crew(s), and started on 2 Nov, seems they should have been able to finish so that you could close by the 30th.
The realtors, title company, and you and your bank/mortgage company and attorney, should have more than sufficient time from the earnest money contract signing on 24 Oct to have everything ready for closing on 30 Nov, soley awaiting completion of work.
You don't want a lawyer, you want to bluff them to change the terms you agreed were fair.
We find for the seller.