Imo the Risco T/O offer is a ruse. They had to make a takeover...

Imo the Risco T/O offer is a ruse. They had to make a takeover offer to increase their shareholding, but must have known that NGP held over 25% and would not sell at the offered prices. Why did they bother?
They knew they would not reach the 90% threshold which would enable a compulsory bid, and that they would not reach the 75% threshold which would enable them to pass special resolutions, both of which may have been worth the effort.

At the 50% threshold, they can pass ordinary resolutions.....
Source : aliens.com.au
A person who owns or has voting control over more than 50% of a company’s shares can unilaterally pass an ordinary resolution, because it requires approval by a majority of votes cast. Importantly, directors can be appointed and removed by shareholders by ordinary resolution. (In practice, a person can normally pass an ordinary resolution on their own with less than a 50% interest given voter turnout at company meetings is often substantially lower than 100%.)
(Note: where there remain minority shareholders in a company, the company’s directors cannot favour the controlling shareholder over the others because the directors have a duty to consider the interests of the company as a whole. Further, related party dealings that require shareholder approval will likely need to be approved by the minority shareholders alone, with the controlling shareholder(s) excluded from voting.)

A clue imo is the unusual (imo) agreement reached that Risco may appoint a Director if their shareholding reaches 30% and they are looking at ASX guidance which enables an entity to be removed from the ASX if "excluding the bidder and its related bodies corporate, the number of target shareholders having holdings with a value of at least $500 is less than 150". In their Bidders statement, Risco state that in such circumstances they intend removing TAP from official list of the ASX.

Whatever the reason, it is unlikely to be beneficial to us "small" shareholders and conversely worth of a premium to Risco.

I found TAP have a provision of US$25,2m for decommissioning of wells in their financials. The valuation adds another $8,2m to this in reaching the 9,1c recommendation. Why the 30% addition by Risco which comes off the bottom line?

I still consider 13c to be fair present value and that control is worth a premium.