I think it's wrong to mix 158s and 170s; they come from a different times, and were to meet different requirements.
BR's programme to replace first generation DMUs started with 'Pacers'; quite simply. they were all that could be afforded at the time and could be got through the investment criteria laid down by government. The first batch of 142s/143s were ordered in June, 1984, but by then an invitation to tender had been issued to BREL and Met-Camm (in April 1984) for up to 50x2-car DMUs, similar to the 150 and 151 prototypes which had already been procured, and tenders had been returned for these before the 142s/143s were ordered. Therefore, at the time the contracts for 'Pacers' were placed, BR already knew that ''Sprinters' would cost roughly 1/3 more, and the 150/1s were subsequently ordered in August of the same year.
Further authorities were obtained to procure more 'Sprinters', and the invitation to tender went out for 20m long vehicles, with 1/3-2/3 doors; however, after these were received, there was a change of mind by the Provincial Sector and the bidders were asked to revise their bids and submit supplementary offers for 23m long vehicles with end doors. The design submitted by BREL wasn't acceptable to BR's D&MEE, and so 23m long vehicle were ordered from Leyland and Met-Camm (155s and 156s); however, there were some lines were 23m long vehicles would have had clearance problems, and so 150/2s were ordered from BREL primarily for such services.
The Provincial Sector created the Express Network, and initially it was worked by 155s/156s; however, these didn't compare favourably with longer distance InterCity trains, and so the requirement for what became class 158s was developed. At that time, these weren't really seen as being the jack of all trades which they became, and were only intended for either long distance services, or those which required an internal environment similar to those found on IC services.
I don't think they were referred to as class 157s at the time, but BR issued an invitation to tender for more trains similar to class 155s/156s; however, 158s were still being built, and BREL tendered to make more 158s for a lower unit price than offered by other bidders for ones without HVAC, and so more 158s were procured although these weren't BR's first choice from s design point of view. The decision to procure more 158s instead of 155/156 type trains was determined solely by cost.
There were financing problems somewhere along the way, and consequently some of the class 158s ended up with NSE (after being modified at Rosyth) as class 159s; a result of this was that some first generation DMUs remained in the north west and Scotland. The 157s emerged at this time, which was not much before privatisation.
I'm sure that the first images I saw of the class 157s showed end gangways and end doors, and the front end shown in Wikipedia came along later; certainly, if the project had gone ahead, they would have been procured from Hunslet-Barclay, but my understanding was that they would have been assembled at Kilmarnock. Moreover, my understanding was that they would be basically either 158s without HVAC or 155/156 type trains, but I don't know what the planned maximum speed was.
The situation post-privatisation was different to that which existed before; the M&EE departments which each sector of BR had disappeared, and so (with the major exception of Virgin) franchises opted for "platform" products, and Adtranz was the first off the mark in providing these. Moreover, when learning what the post-privatisation market wanted, the views of ROSCOs were taken into account by Adtranz; therefore, the 'Turbostar' could work in multiple with BR's 14X/15X family and operate at Sprinter speeds, and have a flexible internal design so that seat layouts, internal doors, the position/number/types of toilets, etc could all be determined by users/owners.
However, as Adtranz developed three main platforms - 'Turbostar', 'Electrostar', and 'Citystar' - it's possible that the company didn't anticipate that 'Turbostars' would be used on services such as those in Scotland where they are now being replaced by 2+4/5 HSTs in Scotland; 'Citystars' were never ordered - but the design of the 220 nose looks very similar to that on the 'Citystar' tender drawings sent to Virgin by Adtranz!
As 'Turbostars' seem to be very much sought after, Adtranz must have got the design pretty well right for local/regional trains.
But - back to the original question - would Regional Railways have ordered trains like 'Turbostars' or 158s if privatisation hadn't come along?
So far as I am aware, there weren't any plans to increase Regional Railways fleet size, so apart from replacing the last of the "heritage" DMUs, no more would have been bought; therefore, IF more trains had been procured, I guess the design would have been determined by what they were intended for, because as a general rule 1/3-2/3 doors were used for local/suburban services, end doors for longer distance ones. So if they'd been ordered as direct replacements for the "heritage" units operated on suburban services around Manchester or in Scotland, they might have been 'Turbostar' type trains; but it might have been that - to get new trains through the investment process - 150s would have been transferred to those areas, and something like 158s (or 175s) procured to work elsewhere.
But one thing which can be said with certainty; BR would NOT have purchased 185s. There'd have been hell to play if anyone had even suggested procuring DMUs which could not operate, say, between York and Scarborough, at SP speeds.