MW1e: Quick Plausibility Check

Is 26 years plausible for the typical length of a MechWarrior's career?

It's long enough to beget and train an heir, yet not so long that 400+ MechWarriors/year out of Sun Zhang becomes totally implausible. (Multiple factors would give the Combine higher turnover than expected.) So it's definitely the right ballpark, although pushing the long side.

Are 250 combat encounters (half 'Mech-vs-'Mech) per year plausible?

Yes, of course. It's a staple of the setting that 'Mechs often clash without being destroyed. Combining statistics given in BattleForce (1st edition) and the way MW1e awards XP should tell us just how little damage it takes to drive enemy 'Mechs off. (MW1e's Ceti Hussars entry implies that high intensity conflicts don't rise above 10% annual casualties.)

How big is a typical garrison?

Well, there's about 7 times as many 'Mech companies as there are planets in the original setting, but I want to look at 3025 proper. From the old House Steiner sourcebook:

• 42 regiments are assigned to the ~93 worlds within 2 jumps of Kuritan or Marik space. Assuming 12.5 regiments are engaged in assault or relief duty (Steiner's full 75.1 regiments * 4/36 assault & 2/36 relief), spreading the remainder as thinly as possible would give each border world a garrison of 34.2 'Mechs (minus however many have embarked on raids). 
• 16 regiments are assigned to the ~55 worlds within 2 jumps of the periphery. Spreading them as thinly as possible would give each of these outworlds a garrison of 31.4 'Mechs (minus however many have embarked on raids).
• 17 regiments garrison 13 key worlds of the interior.
• More than 140 interior worlds have no 'Mech regiments assigned to their region.
• "Garrison units usually consist of infantry, armored, and artillery regiments, whose personnel are mostly native to the planet. There are usually only a few 'Mech lances that pull garrison duty on a world unless that planet is strategically important."

Steiner could put a lance on every interior world if each of their 75 regiments detached two lances apiece. (This wouldn't break the setting.) However, the Trell system is only 45 light-years from the nearest Drac world, and it didn't get a lance until Oberon's raids became a problem. So the interior worlds probably don't have 'Mech garrisons either.

Page 30 of MW1e says "Unlike an assault... [raids] often involve a significantly smaller force (often of one to two Companies...)" I.e., 12-24 'Mechs is significantly smaller than an assault force. Since an average assault force is about 140% of the defender's mass (page 96), it follows that any world worth conquering will be garrisoned by significantly more than 17-34 'Mechs. The world of Chara III (also called Pacifica) has a battalion in garrison, though apparently that's due to its agriculture, rather than strategic positioning. Maybe it's not worth conquering.

So, based on my earlier figures...

• Key worlds average 1 full battalion, 2 full companies and 6 smaller units. A few add extra full regiments. 
• Theater Headquarter worlds are the same, except that 2/3rds of their forces are engaged in assaults, raids and relief. 
• Strategic worlds and the scant handful of other worlds within 1 jump of the Kurita or Marik borders have 2 full companies and 6 smaller units.
• Worlds in the Periphery theaters or 2 jumps from Kurita or Marik have 1-3 lances to fend off raids. 
• Worlds 3 or more jumps from the border are garrisoned by conventional regiments only.

A few incidental curiosities from the Lyran deployment table:

• CMO 26 (which I read as "Coventry Military Outpost #26") is on the map. CMO 1-25 either don't have 'Mech garrisons or are known by other names. 
• I'm guessing "Ellison IV" should be "Corridan IV."
• Two regiments in the cavanaugh theater are assigned to "Denebola V," which isn't actually in that theater (only three other worlds are). Don't know if the two regiments should be moved to other worlds in the same theater or moved to the other theater.Would have to check enemy deployments before deciding.