I think you are in the (not uncommon position) of being stuck between the incompetence of Lambeth Housing and the bureaucracy of Lambeth Planning.
(a) My suspicion is that in the current near-crisis management position of the housing department - with half the staff fighting to keep their jobs and proposals for the ALMO transfer dominating both senior staff and councillors' thinking - no one in the department is actively looking at this project.
(b) My experience of Lambeth's planning department is that many planning officers just bung all correspondence on a planning application on the case file, and only start to read it all once the closing date for comments has passed and they have to start summarising the objections for any planning committee hearing. They are just used to objections. Request for information don't get read until far too late in the process.
(((gaijingirl)))
[Warning - paragraphs that follow may contain both rambling planning bits and jaded political cynicism]
What happens to the money from the land sale is strictly not "a relevant planning consideration" - it is just part of the deal between Lambeth Housing and Metropolitan to sell the site at a discount to full market value in return for nomination rights to some/all the tenancies. Presumably any sale to Metropolitan is subject to planning consent.
The Housing Department ought to have kept people on the estate up to date on what was planned with the money. My suspicion is that given the apparent Housing budget shortfall, they are trying to divert as much money from these sales as possible to other housing projects.
This is where you need to force Toren Smith to be challenging his Labour Party colleagues responsible for the Department on where the money is going, as well as pressing the planning department.
Until you have confirmation of whether local environmental improvements on the estate are going to be funded from the land sale, you can't know for sure whether you still need to demand them, either as specific planning conditions to minimise the "loss of amenity" some tenants will suffer, or as a cash contribution for planning gain, secured by a legal section 106 agreement.
So, as a precaution, I'd recommend that you make sure that your comments and questions on the planning application are officially recorded as an objection. Write a follow up letter, copied to the Interim Head of Development Control, saying that in the absence of information to assure you that the scheme will not have an adverse impact on the residential amenity, secure parking facilties, access to play space (or whatever factors are most important to you and your neighbours) you wish this and your earlier comments to be counted as an objection.
Even better, get your neighbours to write in in similar terms. This way there is a greater chance that the overall package of environmental improvements for the estate will get tied down in a legally binding agreement linked to the planning consent, rather than just relying on the word of Lambeth housing officers.
Metropolitan's project budget will have made assumptions about the total amount they have to pay for site purchase, on-site environmental improvements and any section 106 obligations. My experience is that Housing Associations seldom expect to have to stump up section 106 cash.
I haven't spoken to anyone from Friends of Brockwell Park, but I suspect they'll be asking for Metropolitan to put some money towards a more appropriate boundary fence/hedge with the park. This could be in competition with the estate for a share of the pot.
It should be possible to get a lot of of this thrashed out by "political" intervention before any planning meeting, through a public and non-deniable public commitment by the Council that the housing department will be paying for most of the works. But unless tenants are down as registered objectors to the project, there is a danger that the scheme will just get pushed through, because extra social housing is seen as "a good thing".