Of course the Glazers are not going to "willingly feck up the club" as you put it. They are in it for the long haul, as has often been mentioned. Gill has a nice job and appears to be a reasonably competent, respected and, of course, well paid CEO. Fergie realises that retirement is around the corner and wants to go out on a high with perhaps two remianing goals to achieve. One more European Cup to put the club on a par with the other four times and more winners and another league title to put United ahead of Liverpool. He could walk away right now if he wanted to or if he vehemently disagreed with what's going on but those remaining ambitions override any reservations he may have.
The Glazers realise that by 2017 United could be a very valuable investment which they could jetison for real money - maybe close to £2 bn as you suggest. They would like, by that stage, having paid off the PIK debt, to be in a positikon to repay the bond holders or at worst to have to refinance only a part of that debt. This is the prospect as they most probably see it, instead of being forced to sell now for the comparatively measly sum of say £450 net (£1.2bn less the bond less their PIK debt).
If you take the Forbes valuation method then 12x operating profit would mean that such profit would have to be around £167m in 2017 (£100m currently, as we know). By no means impossible judging by the story so far. However, a recent BBC article on Liverpool's valuation suggests that they are valued at 8.5 x. If that is the applicable multiple to value football clubs and it remains so, United will need to produce net profits of closer to £235m in 2017 to give the Glazers what we think they are aiming for.
This presupposes that the debt is paid off or is considerably reduced. At this year end there was cash in the bank. However, a large part of that was/is earmarked to be funnelled through to the Glazers to reduce their PIK debt. They will need more to liquidate that debt and also, perhaps, to build up cash reserves to repay the bondholders in 2017. In addition there will, no doubt be infrastructure spending requirements as well as expenditure on the team.
It's difficult to see how all these objectives are going to be achieved. It seems that real expenditure on the team is on hold for now - simply because overheads cannot be allowed to exceed their current levels (as a % of revenue). However, with the retirements of Giggs, Scholes, Neville, VDS and maybe Rio too, suitable replacements will have to be found. This will be a combination of say the young hopefuls which Fergie has already brought on board plus two or three new signings, whose combined wages won't exceed those of the old stagers.
The problem is, whilst there are restraints on spending on the team, what about United's competitiveness in the short term and what effects will under performance have on revenues and on the whole Glazer plan for the future ?
